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Field Evaluation of Timber Preservation Treatments for Highway Applications Final Report
Field Evaluation of Timber
Preservation Treatments for
Highway Applications
Final Report
December 2007
Sponsored by
the Iowa Department of Transportation (CTRE Project 06-252)
and
the Iowa Highway Research Board (IHRB Project TR-552)
Iowa State University’s Center for Transportation Research and Education is the umbrella organization for the following centers and programs: Bridge Engineering Center • Center for Weather Impacts on Mobility
and Safety • Construction Management & Technology • Iowa Local Technical Assistance Program • Iowa Traffic Safety Data Service • Midwest Transportation Consortium • National Concrete Pavement
Technology Center • Partnership for Geotechnical Advancement • Roadway Infrastructure Management and Operations Systems • Statewide Urban Design and Specifications • Traffic Safety and Operations
About the Bridge Engineering Center
The mission of the Bridge Engineering Center is to conduct research on bridge technologies to
help bridge designers/owners design, build, and maintain long-lasting bridges.
Disclaimer Notice
The contents of this report reflect the views of the authors, who are responsible for the facts
and the accuracy of the information presented herein. The opinions, findings and conclusions
expressed in this publication are those of the authors and not necessarily those of the sponsors.
The sponsors assume no liability for the contents or use of the information contained in this
document. This report does not constitute a standard, specification, or regulation.
The sponsors do not endorse products or manufacturers. Trademarks or manufacturers’ names
appear in this report only because they are considered essential to the objective of the document.
Nondiscrimination Statement
Iowa State University does not discriminate on the basis of race, color, age, religion, national
origin, sexual orientation, gender identity, sex, marital status, disability, or status as a U.S.
veteran. Inquiries can be directed to the Director of Equal Opportunity and Diversity,
(515) 294-7612.
Technical Report Documentation Page
1. Report No.
2. Government Accession No.
CTRE Project 06-252
IHRB Project TR-552
4. Title and Subtitle
Field Evaluation of Timber Preservation Treatments for Highway Applications
3. Recipient’s Catalog No.
5. Report Date
December 2007
6. Performing Organization Code
7. Author(s)
Jake J. Bigelow, Carol A. Clausen, Stan T. Lebow, and Lowell Greimann
8. Performing Organization Report No.
9. Performing Organization Name and Address
Center for Transportation Research and Education
Iowa State University
2711 South Loop Drive, Suite 4700
Ames, IA 50010-8664
10. Work Unit No. (TRAIS)
12. Sponsoring Organization Name and Address
Iowa Highway Research Board
Iowa Department of Transportation
800 Lincoln Way
Ames, IA 50010
11. Contract or Grant No.
13. Type of Report and Period Covered
Final Report
14. Sponsoring Agency Code
15. Supplementary Notes
16. Abstract
Timber material repair and replacement cost for timber bridges is a considerable expense to highway agencies in Iowa, especially to
county road departments. To address these needs, the objectives of this investigation was to study the field effectiveness of various
treatment alternatives used on Iowa roadway projects and to determine if the current specifications and testing are adequate for
providing proper wood preservation. To satisfy the research needs, the project scope involved a literature review, identification of
metrics, questionnaire survey of Iowa counties, onsite inspections, and a review of current specifications and testing procedures. Based
on the preservative information obtained, the following general conclusions were made: Copper naphthenate is recommended as the
plant-applied preservative treatment for timber bridges. Best Management Practices should be followed to ensure quality treatment of
timber materials. Bridge maintenance programs need to be developed and implemented. The Iowa Department of Transportation
specifications for preservative treatment are the regulating specification for bridges constructed with state or federal funding in Iowa
and are also recommended for all other bridges.
17. Key Words
preservatives—specifications—testing—timber —treatment —wood
18. Distribution Statement
No restrictions.
19. Security Classification (of this
report)
Unclassified.
21. No. of Pages
22. Price
104
NA
Form DOT F 1700.7 (8-72)
20. Security Classification (of this
page)
Unclassified.
Reproduction of completed page authorized
FIELD EVALUATION OF TIMBER PRESERVATION
TREATMENTS FOR HIGHWAY APPLICATIONS
Final Report
December 2007
Principal Investigator
Terry J. Wipf
Director, Bridge Engineering Center
Center for Transportation Research and Education, Iowa State University
Co-Principal Investigator
F. Wayne Klaiber
Professor, Department of Civil, Construction, and Environmental Engineering
Iowa State University
Authors
Jake J. Bigelow, Carol A. Clausen, Stan T. Lebow, and Lowell Greimann
Sponsored by
the Iowa Highway Research Board
(IHRB Project TR-552)
Preparation of this report was financed in part
through funds provided by the Iowa Department of Transportation
through its research management agreement with the
Center for Transportation Research and Education,
CTRE Project 06-252.
Center for Transportation Research and Education
Iowa State University
2711 South Loop Drive, Suite 4700
Ames, IA 50010-8664
Phone: 515-294-8103
Fax: 515-294-0467
www.ctre.iastate.edu
TABLE OF CONTENTS
ACKNOWLEDGMENTS ............................................................................................................ XI
EXECUTIVE SUMMARY ........................................................................................................XIII
1. GENERAL...................................................................................................................................1
1.1. Introduction...................................................................................................................1
1.2. Research Objectives......................................................................................................1
2. BACKGROUND .........................................................................................................................3
2.1. Protection of Timber Bridges .......................................................................................3
2.2. Decay Mechanisms in Timber Bridges.........................................................................3
2.3 Application of Preservatives..........................................................................................4
2.4 Timber Pile Research.....................................................................................................5
3. USE OF PRESERVATIVES IN IOWA ......................................................................................6
3.1. State Specification requirements ..................................................................................6
3.2. Iowa County Survey Results.........................................................................................6
3.3. Field Investigation ......................................................................................................11
4. PLANT-APPLIED PRESERVATIVE TREATMENTS ...........................................................13
4.1. Oilborne Preservatives................................................................................................13
4.2. Waterborne Preservatives ...........................................................................................44
4.3. Plant-Applied Preservative Summary.........................................................................51
5. IN-PLACE PRESERVATIVES TREATMENTS .....................................................................57
5.1. Surface Treatments .....................................................................................................57
5.2. Pastes ..........................................................................................................................58
5.3. Diffusible Chemicals ..................................................................................................58
5.4. Fumigants....................................................................................................................59
5.5. In-Place Preservative Summary..................................................................................60
6. INSPECTION TOOLS AND TESTING ...................................................................................62
6.1. Visual Assessment ......................................................................................................62
6.2. Probing & Pick Test....................................................................................................63
6.3. Moisture Measurement ...............................................................................................63
6.4. Sounding .....................................................................................................................63
6.5. Stress Wave Devices...................................................................................................64
6.6. Drill Resistance Devices.............................................................................................64
6.7. Core Boring.................................................................................................................64
v
6.8. Preservative Retention Analysis .................................................................................64
7. SPECIFICATIONS AND GUIDELINES .................................................................................66
7.1. Iowa Department of Transportation State Specification.............................................66
7.2. American Wood Protection Association Standards (AWPA) ....................................68
7.3. American Institute of Timber Construction (AITC)...................................................70
7.4. Best Management Practices for Use of Treated Wood in Aquatic Environments......71
7.5. Specification Summary...............................................................................................72
8. SUMMARY, CONCLUSIONS AND RECOMMENDATIONS..............................................74
CITED REFERENCES..................................................................................................................76
ADDITIONAL REFERENCES.....................................................................................................78
APPENDIX A..................................................................................................................................1
vi
LIST OF FIGURES
Figure 3-1. Iowa counties timber usage in new construction (63 respondents) ..............................7
Figure 3-2. Iowa county plant-applied preservative usage (40 respondents) ..................................8
Figure 3-3. Iowa counties in-place preservative treatment methods (63 respondents) ...................9
Figure 3-4. Iowa counties expected life of timber bridge components (63 respondents)..............10
Figure 3-5. Inspection methods used by Iowa counties (56 respondents) .....................................11
Figure 3-6. Iowa counties and preservatives inspected .................................................................12
Figure 4-1. Good piles kept above and back from stream channel lasted longer than other pile
locations .............................................................................................................................15
Figure 4-2. Poor piles with suspected improper treatment in contact with constantly moist ground16
Figure 4-3. Comparison of different aged bridge piles located in stream channel; all piles in good
condition ............................................................................................................................16
Figure 4-4. Common visual signs of interior decay of poor piles located in stream channels ......17
Figure 4-5. Creosote bleeding on the sun exposed side of pile can be minimized by vacuum,
steaming, or expansion bath during post-treatment process ..............................................17
Figure 4-6. Breaks in preservative barrier by exterior damage leads to premature decay
(a)Mechanical damage, (b)Debris damage, (c)Fire damage, (d) Weathering damage ......18
Figure 4-7. New piles showing good preservative penetration of sapwood..................................18
Figure 4-8. Exposed end grain provides direct path for infiltration of decay and heavy
weathering..........................................................................................................................19
Figure 4-9. Good metal pile cover to prevent pile top decay ........................................................19
Figure 4-10. Good cap cover provides moisture protection and extends life of the member........20
Figure 4-11. Good individually treated multiple member cap beams allow better seasoning of
wood prior to treatment and better penetration..................................................................20
Figure 4-12. Poor pile cap with exposed end grain and decay ......................................................21
Figure 4-13. Members must be properly seasoned prior to treatment to avoid unwanted checking
and associated deterioration...............................................................................................21
Figure 4-14. Good backwall with treated end grain ......................................................................22
Figure 4-15. Poor end grain with decay was most common defect found in backwalls................22
Figure 4-16. Mechanical damage and physical defects have exposed possible untreated wood ..23
Figure 4-17. Backwalls performed well in highly moist area with little visible decay .................23
Figure 4-18. Good performing wing wall member with good preservative retention...................24
Figure 4-19. Good interior stringers protected from moisture and sunlight by deck ....................24
Figure 4-20. Good stinger end grain treatment with no physical defects or decay .......................25
Figure 4-21. Typical checking seen on exterior stringers due to seasoning and direct sunlight ...25
Figure 4-22. Poor exterior stringer with creosote bleeding and severe split. ................................26
Figure 4-23. Typical physical defects at end grain........................................................................26
Figure 4-24. Good end grain preservative treatment showing less physical defects and decay with
a protective wearing surface ..............................................................................................26
Figure 4-25. Nail laminated deck with individually treated 2x4’s on edge showed very good
performance and good preservative penetration................................................................27
Figure 4-26. Poor decking with mechanical damage allowing water to penetrate preservative
barrier or pool in cracks leading to decay..........................................................................27
Figure 4-27. Pooling water allowed brown rot to grow on mechanically damaged deck surface.28
Figure 4-28. Creosote visibly migrating upward through wearing surface presents environmental
concerns and possible tire traction issues ..........................................................................28
vii
Figure 4-29. Good cap beam with complete preservative barrier..................................................30
Figure 4-30. Good end grain treatment prevents decay and gives good dimensional stability .....30
Figure 4-31. Good cap beam with seasoning prior to treatment allowing preservative to infiltrate
longitudinal checks creating a complete preservative barrier............................................31
Figure 4-32. Post treatment seasoning and checking creates avenues for decay to reach untreated
wood...................................................................................................................................31
Figure 4-33. Good wingwall with very little ultraviolet degradation and no treatment bleeding. 32
Figure 4-34. Backwall with good in-service condition kept high and away from stream channel32
Figure 4-35. Good interior stingers were shaded and protected from moisture by deck above ....33
Figure 4-36. Stringer end grain with good treatment and no visible decay...................................33
Figure 4-37. Satisfactory stringer with checking at incising marks ..............................................34
Figure 4-38. New exterior string with seasoning checks forming on the surface .........................34
Figure 4-39. Treated bridge deck with excellent preservative treatment and member condition .35
Figure 4-40. Underside of deck in good condition with no visible defects or bleeding................35
Figure 4-41. Mechanical damage caused by spacing members apart; deck is screwed down which
helped prevent rocking of planks and severe damage .......................................................36
Figure 4-42. Physical defects at endgrain, ca. 1980 ......................................................................36
Figure 4-43. Good guard railings (a) placed 2006 (b) placed 1988...............................................37
Figure 4-44. Railing post with field cut end grain and no in-place treatment which increased the
amount of physical defects.................................................................................................37
Figure 4-45. Copper naphthenate treated cap beam with building felt cover for protection from
nesting animals ..................................................................................................................39
Figure 4-46. End of pile cap with building felt cover providing protection from moisture and
weathering..........................................................................................................................39
Figure 4-47. Good copper naphthenate backwall ..........................................................................40
Figure 4-48. Good end grain treatment..........................................................................................40
Figure 4-49. Good copper naphthenate-treated stringers ..............................................................41
Figure 4-50. Good exterior stringer with no physical defects or excess preservative bleeding ....41
Figure 4-51. Exterior stinger with checking due to in-place seasoning ........................................42
Figure 4-52. Top side of cantilevered copper naphthenate-treated decking..................................42
Figure 4-53. Good end grain of copper naphthenate-treated bridge deck .....................................43
Figure 4-54. Good treatment retention on underside of deck ........................................................43
Figure 4-55. a) Guard rail post with mechanical damage and no decay present; b) close up of
damaged area .....................................................................................................................46
Figure 4-56. a) Reused CCA guard rail post; b) close up of guard rail post .................................46
Figure 4-57. Top of a guard rail post showing end grain physical defects....................................47
Figure 4-58. New ACZA treated back wall planks properly seasoned prior to treatment ............48
Figure 4-59. ACZA treated decking and exterior stringer in good condition with only minor
physical defects..................................................................................................................48
Figure 4-60. ACQ treated lumber used as guard railing members ................................................50
Figure 4-61. ACQ railing post with splitting that is generally associated with waterborne
treatments...........................................................................................................................50
Figure 4-62. Average ratings of stakes when treated with retentions for structurally critical
structures in ground contact...............................................................................................53
Figure 4-63. Average ratings of stakes when treated with retentions intended for piling.............54
Figure 4-64. Good wearing surface protects the timber decking...................................................56
viii
LIST OF TABLES
Table 4-1. Properties and uses of plant-applied preservatives for timber bridges.........................52
Table 4-2. Estimated service life of treated round fence post in southern Mississippi .................55
Table 5-1. Properties and uses of in-place preservatives for timber bridges.................................61
Table 7-1. Minimum preservative retention requirements ............................................................66
Table 7-2. Minimum preservative penetration requirements.........................................................67
Table 7-3. AWPA Use Category and Commodity Specifications for timber bridge elements .....69
Table 7-4. AWPA and Iowa DOT specification preservative penetration requirements ..............73
Table 7-5. AWPA and Iowa DOT specifications preservative retention requirements.................73
ix
ACKNOWLEDGMENTS
This research project was sponsored by the Iowa Department of Transportation and the Iowa
Highway Research Board. The authors would like to thank the Technical Advisory Committee
and the County Engineers for their participation in the questionnaire, and assistance in
identifying preservative treatments used on their timber bridges. The authors would like to thank
Matt Schwarzkopf for his help with completing several of the tasks involved with this project
including the field investigations. Thanks to Mike LaViolette for his help with putting together
survey information. Thanks to Vanessa Goetz with the Iowa Department of Transportation for
her help with specification information. Special thanks to Boone, Bremer, Butler, Clinton,
Delaware, Lyon, Osceola, and Sioux County Engineers for their help in identifying the bridges
investigated and providing past inspection data for these bridges.
xi
EXECUTIVE SUMMARY
Timber can often be a cost-effective building material for new bridge construction. The
durability of the bridge is greatly dependent upon proper attention to construction details and
fabrication, as well as proper preservative treatment before, during, and after construction. The
repair and replacement cost of deteriorated or damaged material is a considerable expense to
highway agencies in Iowa, especially to county road departments. To address these needs, the
primary objective of this investigation was to evaluate the performance of different wood
preservatives in the field and to review current specifications and testing procedures to determine
if they provide the level of timber treatment required for acceptable performance.
The Iowa State University Bridge Engineering Center (BEC), in conjunction with the United
States Department of Agriculture Forest Products Laboratory (FPL), evaluated the various types
of wood preservatives used in Iowa. To encompass all aspects of timber bridge preservatives
and to obtain comprehensive conclusions, several variables were studied during the evaluation
processes including preservative type, age, exposure condition, bridge element location,
engineering properties, environmental information, and handling issues. To satisfy these
research needs, the project scope involved a literature review, identification of metrics, a
questionnaire survey of Iowa counties, on site inspections, and a review of current specifications
and testing procedures.
Based on the preservative information obtained the following general conclusions were made in
regards to timber bridge preservative performance. Copper naphthenate is recommended as the
plant-applied preservative treatment for timber bridge elements. Copper naphthenate has been
tested extensively by the FPL and has good handling characteristics, clean surfaces, and
comparable availability. During the construction of timber bridges the Best Management
Practices should be followed to minimize environmental impacts to the surrounding ecosystem
and ensure quality treatment of both plant-applied and in-place preservatives. Timber bridge
maintenance programs need to be developed and implemented and should include routine
inspections, evaluations, routine in-place treatment applications, and data management for fleets
of timber bridges. Although the American Wood Protection Association standards are the basis
for the specifications, the Iowa Department of Transportation specifications for preservative
treatment are the regulating specifications for bridges constructed with state or federal funding in
Iowa and are also recommended for all other bridges.
xiii
1. GENERAL
1.1. Introduction
Timber can often be a cost-effective building material for new bridge construction. The
durability of the bridge is greatly dependent upon proper attention to construction details and
fabrication, as well as proper preservative treatment before, during, and after construction. The
life span of existing timber bridges can also be increased with careful attention to design or
construction details during field inspection. The use of timber in transportation structures (e.g.,
bridge superstructures and substructures, abutment retaining walls, guardrail components, etc.) is
common in Iowa. Unfortunately, premature deterioration of these timber components is also a
common problem.
In some cases, problems occur due to inadequate attention to construction details that can lead to
moisture problems regardless of the type of treatment used prior to construction. In other cases,
the particular treatment method used may be incorrect. Various products are currently being
used for the treatment of wood materials in Iowa; however, creosote has been the most common
choice for treatment due to is proven performance and availability. Recently changing
environmental concerns and public perception has made creosote less available and more
expensive for bridge owners in the state of Iowa. Other products recently used in Iowa, such as
copper naphthenate, have just recently been used as a creosote replacement.
There is existing performance data on all preservative systems at various retention levels and in
various wood species. However, most of these data are on samples at either 100 percent
penetration or a complete envelope treatment. In practice, wood preservatives provide excellent
barriers against deteriorating agents (e.g. fungi and insects), but performance can be
compromised in applications requiring field fabrication (Silva et al. 1999). Timber bridge
installations frequently involve cutting members, driving spikes into decking, and/or drilling
holes, which are all common causes of treatment barrier compromise that may affect
performance and long-term durability.
In summary, timber material repair and replacement cost is a considerable expense to highway
agencies in Iowa, especially to county road departments. There is a need to study the field
effectiveness of various treatment alternatives used on Iowa roadway projects and to determine if
the current specifications and testing are adequate for providing proper wood preservation. This
report provides bridge owners and engineers with information on the current preservative
treatments, the field effectiveness of preservatives currently being used on Iowa bridges, testing
techniques for preservative evaluation, and the status of current specifications.
1.2. Research Objectives
The primary objective of this research was to evaluate the performance of different wood
preservatives in the field and to review current specifications and testing procedures to determine
if they provide the level of timber treatment required for acceptable performance.
1
The Iowa State University Bridge Engineering Center (BEC), in conjunction with the United
States Department of Agriculture Forest Products Laboratory (FPL), evaluated the various types
of wood preservatives used in Iowa. To encompass all aspects of timber bridge preservatives
and to obtain comprehensive conclusions several variables were studied during the evaluation
processes including the following: preservative type, age, exposure condition, bridge element
location, engineering properties, environmental information, and handling issues.
To satisfy these research needs, the project scope included the following tasks:
1. A literature review was conducted to learn about the preservatives that are available,
preservative properties, and their effectiveness in the field. A brief summary of these
preservations are presented herein.
2. The identification of metrics identified a set of tools for bridge inspectors to use in
order to make educated decisions regarding preservative evaluations.
3. A survey of Iowa counties was completed to obtain information on utilization of timber
bridges, preservatives, factors influencing timber usage, life expectancy,
problematic/successful details, and bridge inspection practices.
4. On site inspections were completed to investigate elements in different counties with
problem and non-problem conditions on which different preservative types have been
employed.
5. The review of specification and testing procedures were compared and evaluated to
determine if Iowa specifications needed additional information or updating.
6. The final conclusion and recommendations were developed from the gathered
information in the previous tasks.
2
2. BACKGROUND
2.1. Protection of Timber Bridges
There is a long history of the use of wood as construction material for road bridges in the United
States. These uses have varied from simple, temporary log bridges to more complex structures
that have remained serviceable for over 150 years. Wood is a natural choice for a construction
material because it is inexpensive, relatively simple to fabricate, and locally available in most
parts of the United States. However, for structures that are expected to last more than a few
years, the susceptibility of wood to biodegradation is a major disadvantage. In the 19th century,
engineers overcame this disadvantage by constructing covered bridges that kept the wood dry
and prevented decay. Many of these covered bridges remain in use today. However, it is not
always practical or economical to build structures in a manner that protects wood from moisture.
Therefore, the need for more durable timber provided the driving force for the development of
the pressure treatment industry in the United States. The successful use of pressure treated
railroad ties led to the pressure treatment of other structural products, such as utility poles, piles,
and bridge timbers. Preservative treated wood, however, faced stiff competition from steel and
concrete for construction of road bridges. By the mid-1930’s the cost of steel bridges became
competitive with treated wood, and steel evolved as the primary construction material, with
reinforced concrete the preferred material for bridge decks. In the 1960’s and 1970’s, the use of
timber bridges was given a boost by the widespread acceptance of preservative treated glulam
beams and more recently by the development of stress-laminated lumber. Timber bridges remain
a viable alternative in many situations, and thousands have been built across the United States in
recent decades (Ritter 1992).
Timber bridges remain cost-competitive, and the single most limiting factor for increased use of
timber bridges continues to be concerns with durability. The durability of timber bridges is
largely a product of the initial preservative treatment used to protect the wood, although
construction practices and maintenance also play an important role. The efficacy of the initial
pressure treatment is a function of the inherent properties of the wood type, the preservative
chemical itself, and the quality of the treatment process (the degree of preservative penetration
and retention achieved during treatment). In some cases the properties of the preservative also
play a role in treatment quality. However, the ability to protect wood is not the only
consideration for a preservative treatment. In recent years concerns about the environmental
impacts of preservative treatments have increased, and this is especially true for treated wood
used in or above aquatic environments. Other factors, such as color, odor and surface cleanliness
may also be important in some applications.
2.2. Decay Mechanisms in Timber Bridges
In most applications for timber bridges, decay fungi are the most destructive organisms. Fungi
are microscopic thread-like organisms whose growth depends on mild temperatures, moisture,
and oxygen. There are numerous species of fungi that attack wood, and they have a range of
preferred environmental conditions. Decay fungi are often separated into three major groups;
3
brown rot fungi, white rot fungi, and soft rot fungi. Soft-rot fungi generally prefer wetter, and
sometimes warmer, environmental conditions than brown or white rot fungi.
Termites follow fungi in order of damage to wood structures in the US. Their damage can be
much more rapid than that caused by decay, but their geographic distribution is less uniform.
Termite species in the US can be categorized by ground-inhabiting (subterranean) or wood
inhabiting (non-subterranean) termites. Most damage in the US is caused by species of
subterranean termites.
Other types of insects such as powderpost beetles and carpenter ants can cause notable damage
in some situations, but their overall significance pales in comparison to decay fungi and termites.
Other organisms, including bacteria and mold can also cause damage in some situations, and
several types of marine organisms degrade wood placed in seawater.
The two greatest factors influencing regional biodeterioration hazard are temperature and
moisture (Highley 1999). The growth of most decay fungi is negligible at temperatures below
36 F and relatively slow at temperatures below 50 F. The growth rate then increases rapidly,
with most fungi having optimum growth rates at between 75 F and 95 F. The natural range of
native subterranean termites is generally limited to areas where the average annual temperature
exceeds 50 F. Decay fungi require a moisture content of at least 20% to sustain any growth, and
higher moisture contents (over 29%) are required for initial reproduction (Highley 1999). Most
brown and white rot decay fungi prefer wood in the moisture content range of 40 – 80%. In
almost all cases, wood that is protected from ground contact, precipitation, or other sources of
water will have insufficient moisture to sustain growth of decay fungi. In contrast, wood that is
in contact with the ground often has sufficient moisture to support decay, even in relatively dry
climates. On the other hand, wood can be too wet to support fungal growth. For example, as the
moisture content exceeds 80% void spaces in the wood are increasingly filled with water. The
lack of oxygen and build-up of carbon dioxide in the water limits fungal growth.
2.3 Application of Preservatives
The structure and chemistry of wood affect the ability of preservatives to penetrate into the
wood, as well as the efficacy of some types of preservatives. As a tree develops, new cells that
grow around the outer circumference of the stem form the conductive tissues which comprise the
sapwood. The thickness of the sapwood band varies greatly by species, but in almost all species
the sapwood is the portion of the tree that is most easily penetrated with preservative. The older,
inner sapwood cells eventually stop functioning and form a darker core of non-conductive
tissues called heartwood. In many wood species the heartwood is difficult to penetrate with
preservative.
There are also significant differences between the two broad classes of trees called hardwoods
and softwoods. The wood structure of hardwoods is more complex than that of softwoods and
the differences affect the distribution of some treatments, lessening their effectiveness in
hardwoods. The structure of softwoods is generally simpler and more uniform than that of
hardwoods. Softwoods represent the vast majority of treated wood produced in the US.
4
Even within softwoods and hardwoods there are major anatomical differences between species.
The species group that is most often treated with preservatives is Southern Pine. Southern Pine
trees are characterized by a large sapwood zone that is readily penetrated with most types of
preservatives. In the western US the species most often treated are Douglas-Fir, Ponderosa Pine
and the Hem-Fir species. With the possible exception of Ponderosa Pine, these species tend to
be more difficult to treat with preservatives or demonstrate more variability in their treatability.
Often they must be incised (small slits cut into the wood) in order to obtain adequate penetration.
Another major species group is Spruce-Pine-Fir (SPF). This group contains a large number of
species that grow in the northern United States and Canada. Like the Hem-Fir species, these
wood species tend to be difficult to treat or vary widely in their treatability. The use of treated
hardwoods is largely confined to railroad ties and bridge timbers. Red Oak is the most often
utilized hardwood.
Even though proper preservative treatment creates an excellent barrier against fungi and insects,
the barrier can be compromised during on-site installation or as a result of checks and cracks
from normal weathering and moisture changes. Any break in the treatment barrier may expose
untreated wood to fungal or insect attack (Highley 1999). Although the rate of decay will vary
with the wood species and decay hazard conditions, eventual development of deterioration will
reduce the service life of the structure (Scheffer 1971).
There is a considerable need for periodic inspection and preventative in-place treatments for
timber in bridges (AASHTO 1983; Ritter 1990). Ideally, though not always practical, annual inplace treatment of checks will provide protection from decay. Bridge timbers, like utility poles
and rail sleepers, need to be on an inspection rotation, so that they are periodically inspected for
signs of physical, chemical or biological deterioration. Physical, chemical and biological
deterioration of wood are interrelated and their collective effects need to be considered during a
bridge inspection.
2.4 Timber Pile Research
Past research has been conducted by the BEC on timber abutments that have undergone physical
and biological deterioration (White et al. 2007). The deterioration influences the load carrying
capacity of timber substructures and thus affects the overall performance of the bridge system.
Prior to this work, there was no reliable means to estimate the residual carrying capacity of an inservice deteriorated pile, and thus, the overall safety of the bridge could not be determined. The
lack of a reliable evaluation method can result in conservative and costly maintenance practices
such as replacing the entire substructure system when only one of the piles needs replaced. The
research evaluated procedures for detecting pile internal decay using nondestructive ultrasonic
stress wave techniques, correlated nondestructive ultrasonic stress wave techniques to axial
compression tests to estimate deteriorated pile residual strength, and evaluated load distribution
through poor performing timber substructures by instrumenting and load-testing the abutments of
six in service bridges. The research also evaluates selected rehabilitation, strengthening, and
replacement techniques for timber pile substructure components or entire substructures.
5
3. USE OF PRESERVATIVES IN IOWA
Highway applications of timber material in Iowa vary greatly from bridge pilings, abutment
backwalls, guardrail posts, bridge deck planking and many others. Currently, various in-plant
preservative treatments are being used in Iowa to extend the service life of structures. Creosote
has been the in-plant preservative of choice for many years, however, due to environmental
concerns and handling issues a movement is being made away from creosote to other
preservative alternatives. Additionally, remedial or in-place preservative treatments have seen
minimal usage in the state of Iowa. As Iowa’s timber bridges become older, the implementation
of in-place treatments will be necessary to reduce future costly repair and replacement.
3.1. State Specification requirements
The Iowa Department of Transportation Standard Specifications with GS-01013 Revisions have
several divisions that mention timber products used for timber bridge structures. Division 41
states that creosote, pentachlorophenol, copper naphthenate, chromated copper arsenate (CCA)
and ammoniacal copper zinc arsenate (ACZA) must be used as the specified treatment of timber
bridge elements. The division also requires American Wood Protection Association (formerly
American Wood Preservers’ Association) AWPA standard to be met for preservative retention
and penetration. For the purpose of this report the five DOT specified preservatives will be
discussed in detail plus other recommended preservatives that have been standardized
application in highway construction. The state specification will be discussed further in Chapter
7.
3.2. Iowa County Survey Results
A survey of Iowa’s 99 counties was completed to obtain information on current timber bridge
preservation practices in Iowa. The survey was divided into six categories: utilization of timber
bridges, preservatives used, factors influencing timber usage, life expectancy, problematic or
successful details, and bridge inspection practices. A copy of the survey can be seen in Appendix
A. Sixty three counties responded, in varying degrees, to the survey.
Of the 63 counties that responded approximately 88% utilize timber in their bridge structures and
52% had reservations about using timber for new bridges. Several counties commented that
short lifespan, low durability, and the use of concrete or steel were reasons for their reluctance to
use timber. Counties were also asked if they are constructing or not constructing new bridge
components with timber materials. The results of the new construction usage are shown in
Figure 3-1.
6
60
50
43
42
Number of Counties
40
Yes
No
38
33
30
25
21
20
17
14
10
0
Backwalls & Wingwalls
Pilings & Substructure
Superstructure
Guardrail & Sign Post
New Bridge Components
Figure 3-1. Iowa counties timber usage in new construction (63 respondents)
Forty counties identified the use of plant-applied preservatives. Figure 3-2 shows the resulting
number of counties using a particular plant-applied treatment. Note that several of the counties
use multiple preservatives causing the sum of the preservative usage to be greater than 40.
7
40
35
29
Number of Counties
30
25
20
15
12
9
10
5
5
3
0
0
0
1
0
1
os
ot
e
C
re
C
C
A
Pe
nt
ac
hl
or
op
he
no
C
op
l
pe
rN
ap
ht
he
na
te
ZA
AC
lis
te
d
no
t
C
AC
C
O
th
er
s
op
pe
r
O
xi
ne
O
C
op
pe
r
H
D
AB
C
AC
Q
0
Plant-applied Preservative
Figure 3-2. Iowa county plant-applied preservative usage (40 respondents)
Counties were also questioned on their usage of in-place preservative treatment methods. The
results, shown in Figure 3-3, found very few counties using in-place treatments. One county,
however, stated it used all seven methods listed in Figure 3-3 for in-place preservative treatment.
8
10
9
8
8
Number of Counties
7
6
5
4
3
3
2
1
1
1
1
Boron rods
Flouride rods
Copper boron rods
1
1
0
Liquid fumigants Granule fumigants
Spray applied
Brush applied
In-place Treatment Method
Figure 3-3. Iowa counties in-place preservative treatment methods (63 respondents)
Thirty-two counties identified the specification they use for preservative treatment requirements.
The majority of counties use the Iowa DOT state specification. Listed below is the distribution
of counties using different specifications. Note that none of the 32 counties specify a scheduled
reapplication of the preservative treatment.
•
•
•
•
•
26 counties used state specifications
3 counties used their own county specification
2 counties used specification not listed on the survey form
1 county used AWPA standards
0 counties used AASHTO standards
The counties were asked to rank 11 different disadvantages of timber bridges. The ranking is
listed below with one being the biggest disadvantage.
1.
2.
3.
4.
5.
6.
7.
8.
9.
Durability concerns
Maintenance concerns
Cost
Strength properties
Odor or surface cleanliness (handling concerns)
Difficulty in specifying preservative treatment
Material availability
Ease of installation
Not accustomed to using timber
9
10. Concerns about corrosion of connectors
11. Appearance
The counties were also asked to rank the advantages for using timber bridges. Seven criteria
were given for rating. The ranking is listed below with one being the most advantageous.
1.
2.
3.
4.
5.
6.
7.
Ease of installation
Cost
Material availability
Appearance
Maintenance
Strength properties
Durability
The counties were asked for an estimation of the life expectancy of deck, stringer, piling, and
backwall components. The results are displayed in Figure 4. From Figure 3-4 one can see the
predicted life expectancy of timber decking was highly variable, however, stringers, pilings, and
backwalls were generally expected to last 31 to 50 years.
30
26
25
24
Deck
Stringer
Piling
Backwall
24
Number of Counties
20
15
15
14
12
12 12
11
10
9
8
8
7
7
7
7 7
6 6
5
4
3
3
2
1
1
1
0 0
0 0
1-5 years
6-10 years
0 0
0
11-15 years
16-20 years
21-25 years
26-30 years
31-50 years
Over 50 years
Live Expectancy
Figure 3-4. Iowa counties expected life of timber bridge components (63 respondents)
Counties were asked about their current timber bridge details and if they had problematic or
successful details. Nine counties responded stating they were having good success with their
timber detailing; however, 19 counties responded stating they had problematic details.
10
Lastly, the counties were asked to contribute information pertaining to their inspection and
testing methods. Fifty-six counties have scheduled inspections and of these 56 counties, 40 of
them have a consultant perform the inspections. The counties also identified external and/or
internal inspection and testing methods they perform to determine structural soundness. Internal
detection is used far less than external methods as shown in Figure 3-5. Visual inspection was
the most commonly identified inspection method.
60
53
50
External
Internal
Number of Counties
40
36
35
30
28
20
11
10
1
0
Probing
Other
Pick Test
Drilling/coring
Visual inspection
Sounding
Inspection Method
Figure 3-5. Inspection methods used by Iowa counties (56 respondents)
3.3. Field Investigation
On-site visual inspections were conducted by the research team in counties with problem and
non-problem conditions and with different preservative types. The goal of these inspections was
to evaluate the in-place performance of current preservatives used in Iowa. Creosote,
pentachlorophenol, copper naphthenate, ammoniacal copper zinc arsenate (ACZA), chromated
copper arsenate (CCA) and Alkaline Copper Quaternary (ACQ) were positively identified
preservatives. In total 47 bridges were investigated in eight different Iowa counties. Figure 3-6
shows the counties investigated and the corresponding preservatives investigated.
11
Figure 3-6. Iowa counties and preservatives inspected
When conducting the inspections, all available piles, cap beams, backwalls, stringers, decking,
and guard railing were inspected for decay, physical defects, and damage. Decay, physical
defects, and damage are all signs that the preservative treatment is not performing effectively, or
may have been compromised for future protection. In order to be consistent when inspecting the
bridges a checklist was kept for each bridge. The checklist was based on the FPL rating criteria
for stakes in ground contact, FPL rating criteria for decks above-ground (Crawford et al. 1999),
and the National Bridge Inventory (NBI) condition rating system (US DOT 1995). A summary
of the overall findings and a general assessment of the in field preservative performance follows
in Chapter 4.
12
4. PLANT-APPLIED PRESERVATIVE TREATMENTS
Wood preservatives are expected to protect timber members from attack by a broad range of
organisms without posing significant risks to people or the environment. Preservatives must also
resist weathering and other forms of depletion for extended periods of time. Because of toxicity,
however, many of preservatives are labeled by the Environmental Protection Agency (EPA) as
Restricted Use Pesticides (RUP). The RUP classifications restrict the use of the chemical
preservative, but not the treated wood, to certified pesticide applicators only. The State of Iowa
requires that personnel applying supplemental preservatives to bridges on public property
undergo Pesticide Applicator Training (PAT) and become certified Commercial Pesticide
Applicators under Category 7E (Wood Preservatives). More information on obtaining this
training and certification can be found by contacting the Pest Management and Environment
Program at Iowa State University (http://www.extension.iastate.edu/pme/pat/ or 515-294-1101).
Wood preservatives can be broadly classified as either oilborne or waterborne, based on the
chemical composition of the preservative and the solvent/carrier used during the treating process.
Generally, oilborne preservatives are used with petroleum based solvents ranging from heavy
oils to liquefied gases. Waterborne preservatives are applied using water based solutions such as
water and ammonia (Ritter 1992). There are advantages and disadvantages associated with using
each type that depend upon the application.
Generally, wood preservatives also are categorized by the exposure environment in which they
are expected to provide protection. Ground contact preservatives have sufficient leach resistance
and broad spectrum efficacy to protect wood that is exposed directly to soil and water. Above
ground contact preservatives have intermediate toxicity or leach resistance that allows them to
protect wood that is fully exposed to the weather, but not in contact with the ground. Marine
exposure preservatives have high resistance to decay and marine organism, good leach
resistance, and may require heavy duty treatment (Ibach 1999)
Evaluation of a preservative’s long-term efficacy in all types of exposure environments is not
possible and there is no set formula for predicting exactly how long a wood preservative will
perform in a specific application. When the application is structurally critical, such as a support
member in a bridge, increased retentions are often specified to help ensure durability. Overtreatment, however, may provide little additional durability while increasing the risk of
environmental concerns. The following listing and description of preservatives is not intended to
be exhaustive. The list is limited to preservatives that have been standardized for some type of
application used in highway construction and have been produced commercially.
4.1. Oilborne Preservatives
The most common oilborne preservatives are creosote, pentachlorophenol, and copper
naphthenate. The conventional oilborne preservatives, such as creosote and pentachlorophenol
solutions, have been confined largely to uses that do not involve frequent human contact. The
exception is copper naphthenate, a preservative that has become available more recently but has
been used less widely. Oilborne preservatives may be visually oily, oily to the touch, and
13
sometimes have a noticeable odor. However, the oil or solvent that is used as a carrier makes the
wood less susceptible to cracks and checking and helps prevent moisture movement through the
member. Oilborne preservatives ability to dimensionally stabilize timber members and act as
moisture-barriers, make them the preferred preservative for bridge structural elements (Wacker
and Crawford 2003).
4.1.1. Creosote
Creosote is the oldest and the most common type of oilborne preservative in service today. It
was first patented in 1831 and is produced by the distillation of coal tar or oil shale (Ritter 1992).
Creosote is a chemically complex mixture, and due to variations in the distillation processes
from plant to plant, small differences can be found in the resulting chemical make up. The main
ingredient found in all creosotes, however, is polycyclic aromatic hydrocarbons (PAH). If
leached from the wood, these PAH’s are considered pollutants and can contribute to
environmental concentrations that come from a variety of activities such as motor fuel
combustion, coal burning, and forest fires (Wikipedia 2007). Creosote can be mixed in many
different coal-tar and petroleum solutions. Straight, undiluted creosote, however, is preferred for
most bridge applications due to its higher toxicity to fungi, better penetration properties of both
hardwood and softwood species, and less bleeding.
The small differences in the composition of modern creosotes do not affect their performance as
a wood preservative. Creosoted timber has been found to be effective in most environments
including ground contact, water contact, and above ground locations. The primary uses for
creosote in the past have been for bridge components, utility poles, marine piling, and railroad
ties. Due to its age and extensive use, creosote has a proven record of satisfactory service and
case histories have shown 50-plus years of good in-place service (Ritter 1992). The treated
wood is dark brown to black and has a distinct smoky odor. The preservative does not dissolve
in oil; however, it often has an oily appearance and feel. After treatment, members often have an
oily-like surface residue that causes members to be unpaintable and not ideal for handrails or
places were skin contact is highly probable. The properties of creosote do not accelerate and can
inhibit the corrosion of metal fasteners. Members with fresh creosote surfaces can be ignited and
will burn, however, after a few months of seasoning the volatile parts of the oil components are
gone from the surface and ignition properties are similar to that of untreated wood (Ibach 1999).
As with most oilborne preservatives, creosote is thought to improve the dimensional stability of
the members and causes no noticeable changes in engineering properties.
In the past decade the use of creosote has declined because of handling issues and environmental
concerns. Creosote can easily soil workers clothing and the vapors irritate skin by
photosensitizing exposed areas. However, no health dangers have been found in workers
directly handling and working near treated wood. Sensitive growing plants and foodstuff can be
harmed by creosote vapors and should not be stored with creosoted members in unventilated
areas. The U.S. Environmental Protection Agency (EPA) and treated timber producers have
created Consumer Information Sheets (CIS) with guidance on appropriate handling and site
precautions when using treated wood. For further details an EPA approved CIS can be found in
the Wood Handbook Chapter 14 (Ibach 1999). The EPA classifies creosote solution as a RUP
and can only be handled by state licensed applicators. The use of creosote treated wood,
14
however, is not restricted. Creosote solution is no longer available for use as an in-place
treatment.
4.1.1.1. Creosote Preservative Field Investigations
During the field investigations Iowa counties were found to have wide historic use of creosote
preservative on timber bridges. The bridges had creosoted timbers for piles, cap beams,
abutment backwalls, stringers, and decking, with the oldest elements dating back to 1933. Of
these, the most common creosoted bridge elements were piles. Creosoted piles are still being
used today; however, there has been a decline in the use of creosote for back wall and
superstructure elements. As stated previously this decline is attributed to handling complaints by
the workers and environmental concerns. Creosote piles are still very popular because of their
historical good performance and uncertainty of other newer preservative products. Figures 4-1
through 4-28 display performance issues, both good and poor, associated with bridges
investigated.
Figures 4-1 through 4-9 show creosote bridge piles.
Good abutment
piles
ca. 1933
Dry abutment
location
Figure 4-1. Good piles kept above and back from stream channel lasted longer than other
pile locations
15
Advance decay
& section loss
Constantly moist
crop biomass
Figure 4-2. Poor piles with suspected improper treatment in contact with constantly moist
ground
1993
1943
Wet pile
condition
Figure 4-3. Comparison of different aged bridge piles located in stream channel; all piles in
good condition
16
Longitudinal checking
and splitting
Vegetation growth
indication of decay
Decay infiltration at
cracks
Figure 4-4. Common visual signs of interior decay of poor piles located in stream channels
Excessive creosote
bleeding
Figure 4-5. Creosote bleeding on the sun exposed side of pile can be minimized by vacuum,
steaming, or expansion bath during post-treatment process
17
(a)
(b)
(c)
(d)
Figure 4-6. Breaks in preservative barrier by exterior damage leads to premature decay
(a)Mechanical damage, (b)Debris damage, (c)Fire damage, (d) Weathering damage
Clean surface
Preservative
penetration
Figure 4-7. New piles showing good preservative penetration of sapwood
18
Heartwood
Untreated
sapwood
Treated
barrier
Figure 4-8. Exposed end grain provides direct path for infiltration of decay and heavy
weathering
Figure 4-9. Good metal pile cover to prevent pile top decay
19
Figures 4-10 through 4-13 show creosote bridge cap beams.
Metal top
cover
Metal end
grain
Figure 4-10. Good cap cover provides moisture protection and extends life of the member
Exuding
creosote
Deteriorated
building felt
cover
No physical
defects
Figure 4-11. Good individually treated multiple member cap beams allow better seasoning
of wood prior to treatment and better penetration
20
End Grain
Decay
Damaged
building felt
cover
Figure 4-12. Poor pile cap with exposed end grain and decay
Longitudinal
checking
Figure 4-13. Members must be properly seasoned prior to treatment to avoid unwanted
checking and associated deterioration
21
Figures 4-14 through 4-18 show creosote bridge backwalls.
Good end
grain treatment
Figure 4-14. Good backwall with treated end grain
End grain
decay
Figure 4-15. Poor end grain with decay was most common defect found in backwalls.
22
Physical
defects
Severe mechanical
damage
Figure 4-16. Mechanical damage and physical defects have exposed possible untreated
wood
Piles severely
decayed
No decay
on backwall
Constant moist
area
Figure 4-17. Backwalls performed well in highly moist area with little visible decay
23
Residual
creosote
Highly weathered
graining
Figure 4-18. Good performing wing wall member with good preservative retention
Figures 4-19 through 4-22 show creosote bridge stringers.
No longitudinal
checking
Water stains
w/ no signs of
decay
Figure 4-19. Good interior stringers protected from moisture and sunlight by deck
24
Stringer
end grain
Stringer
bearing
Figure 4-20. Good stinger end grain treatment with no physical defects or decay
Longitudinal
checking
Ultraviolet
discoloring
Figure 4-21. Typical checking seen on exterior stringers due to seasoning and direct
sunlight
25
Figure 4-22. Poor exterior stringer with creosote bleeding and severe split.
Figures 4-23 through 4-28 show creosote bridge decks.
Cupping
Shake
Splitting
Checkin
g
Figure 4-23. Typical physical defects at end grain
Wearing surface
End grain
treatment
Figure 4-24. Good end grain preservative treatment showing less physical defects and
decay with a protective wearing surface
26
Uniform treatment
No physical defects
Figure 4-25. Nail laminated deck with individually treated 2x4’s on edge showed very good
performance and good preservative penetration
Severe
splitting
Surface
abrasion
Figure 4-26. Poor decking with mechanical damage allowing water to penetrate
preservative barrier or pool in cracks leading to decay
27
Figure 4-27. Pooling water allowed brown rot to grow on mechanically damaged deck
surface
Preservative migration
to surface
Figure 4-28. Creosote visibly migrating upward through wearing surface presents
environmental concerns and possible tire traction issues
4.1.2. Pentachlorophenol
Pentachlorophenol (penta) was first patented in 1935 and has been widely used in the United
States since the 1940’s. Penta is the first synthetically produced preservative which allowed its
production to meet industry demand for preservatives. Currently, penta is used to treat
approximately 30 percent of the preservative protected wood each year in the United States.
Poles, posts, and timbers have been the primary timber elements penta has been used to treat
(Ritter 1992).
28
Chlorinated phenols are the active ingredients of penta and are highly effective at preventing the
decay organisms from obtaining energy from the wood (Ritter 1992). At one time, dioxins were
also found in trace amounts in penta, but this problem has largely been overcome. Dioxins are
organic compounds that have been found to bioaccumulate in animals and are suspected
carcinogens (Wikipedia 2007). Typically, penta is dissolved in an organic solvent that acts as a
carrier during the treatment process. The two most common solvents are Type A and C. These
solvents, which are described below, have been found to heavily influence the preservative
performance of the treated wood and should be carefully chosen for the specific field
applications. Penta-based preservatives do not accelerate the corrosion of metal fasteners and do
not cause a change in the engineering properties of the treated wood.
Type A solvents are generally heavy oils and are recommended for bridge structural members
including glue-laminated beams and foundation pilings. Penta in heavy oil is effective when
used in ground contact, freshwater, and above-ground applications but not in marine
environments. Members treated in heavy oil penta have a brown color and may have an oily
surface that is difficult to paint and should not be used in locations were human, plant, or animal
contact is likely. Some odor does occur with this treatment however it is generally associated
with the solvent. The effectiveness of Type A penta is similar to creosote in protecting both
hardwoods and soft woods. Solution temperature and length of pressure periods can allow penta
to penetrate woods that are otherwise difficult to treat. Penta in heavy oil can improve the
dimensional stability of the treated wood.
Type C penta uses light petroleum oil as the solvent carrier. Type C penta is primarily used for
glue-laminated lumber were the lumber is treated prior to gluing and can be used in applications
where human contact is likely. Type C penta has similar treatment characteristics as Type A
penta. Type C penta can penetrate difficult to treat species and does not accelerate corrosion.
The surface of Type C treated wood is paintable and provides some protection from weathering,
however, the protection is not sustained over time (Ritter 1992). Timber that has been treated
with Type C penta should only be used in above ground applications.
All types of penta chemicals are classified by the EPA as RUPs and can only be plant applied by
licensed applicators. The EPA also has set limitations on the amount of dioxins that can be
present in penta. Due to its toxicity humans should avoid excessive contact with the solution and
vapor. The EPA and treated timber producers have created Consumer Information Sheets (CIS)
with guidance on appropriate handling and site precautions when using treated wood. For
further details an EPA approved CIS can be found in the Wood Handbook Chapter 14 (Ibach
1999).
4.1.2.1. Pentachlorophenol Preservative Field Investigations
The field investigations revealed that only flat sawn penta treated timber elements were being
used for bridge construction. No bridge piles were investigated. Specifically, cap beams,
abutment backwalls, stringers, decking, and guard railing were all seen. The range in age of
penta treated material was about 25 years with the earliest bridges dating back to the early
1980’s and the most recent bridge constructed in 2006. In some counties entire new bridges
were being constructed of penta-treated wood while other counties are only using penta treated
29
wood for repair of their existing timber bridge fleet and constructing new bridges with alternate
material. Several counties commented they preferred the use of penta over creosote due to better
handling issues and less preservative bleeding. Figures 4-29 through 4-44 show good and bad
bridge elements treated with penta.
Figures 4-29 through 4-32 show pentachlorophenol bridge cap beams.
Shaded
No physical
defects and good
preservative
retention
Exposed
Figure 4-29. Good cap beam with complete preservative barrier
Minimal physical
defects
Good preservative
retention
Figure 4-30. Good end grain treatment prevents decay and gives good dimensional stability
30
Checking prior
to treatment
Figure 4-31. Good cap beam with seasoning prior to treatment allowing preservative to
infiltrate longitudinal checks creating a complete preservative barrier
No end grain
or top cover
In-service
checking
Figure 4-32. Post treatment seasoning and checking creates avenues for decay to reach
untreated wood
31
Figures 4-33 and 4-34 show pentachlorophenol bridge backwalls.
Good weathering
protection
Figure 4-33. Good wingwall with very little ultraviolet degradation and no treatment
bleeding.
Figure 4-34. Backwall with good in-service condition kept high and away from stream
channel
32
Figures 4-35 through 4-38 show pentachlorophenol bridge stringers.
Good preservative
retention
No physical
defects
ca. 1980
Figure 4-35. Good interior stingers were shaded and protected from moisture by deck
above
Figure 4-36. Stringer end grain with good treatment and no visible decay
33
Incising
marks
Ultraviolet
discoloration
Figure 4-37. Satisfactory stringer with checking at incising marks
Checking
Figure 4-38. New exterior string with seasoning checks forming on the surface
34
Figures 4-39 through 4-42 show pentachlorophenol bridge decking.
Figure 4-39. Treated bridge deck with excellent preservative treatment and member
condition
Figure 4-40. Underside of deck in good condition with no visible defects or bleeding
35
Cracking &
splintering
Figure 4-41. Mechanical damage caused by spacing members apart; deck is screwed down
which helped prevent rocking of planks and severe damage
Figure 4-42. Physical defects at endgrain, ca. 1980
36
Figures 4-43 and 4-44 pentachlorophenol bridge guard railing.
(a)
(b)
Figure 4-43. Good guard railings (a) placed 2006 (b) placed 1988
Treatment
barrier
Seasoning
cracks
Penetration at
incising marks
2006
Figure 4-44. Railing post with field cut end grain and no in-place treatment which
increased the amount of physical defects
4.1.3. Copper Naphthenate
Copper naphthenate has been commercially available since the 1940’s and formulations were
added to the AWPA standards in 1948. Copper naphthenate is the product of the reaction
between petroleum derived naphthenic acids and copper salts. Copper naphthenate has low
animal toxicity allowing it to be purchased in small quantities at retail hardware stores and
lumber yards for in-place treatment (Brient et al. 2004). Copper naphthenate can be dissolved in
a variety of solvents similar to pentachlorophenol, however, AWPA only has standards for heavy
37
oil solvents. With the use of lighter oils copper naphthenate can penetrate difficult to treat wood.
No standards have been developed for treatment of hardwood species except for railroad ties.
Copper naphthenate-treated wood is bright green colored and weathers to a light brown. Freshly
treated wood has an odor that can dissipate over time. Copper naphthenate is effective for use in
ground contact, water contact, and above ground applications. It is not however, standardized
for saltwater applications. The most common use has been for utility poles, but it is becoming
popular for structural lumber, post, and glulam beams due to the clean surface and resistance to
in-service bleeding (Wacker 2003). The clean surface of copper naphthenate-treated wood can
be painted, however, the paintability depends on the solvent, treatment procedures, and the time
allowed for the member to cure properly. Similar to other oilborne preservatives, in-place
dimensional stability is enhanced, corrosion of metal fasteners is not significantly increased, and
engineering properties are unchanged with proper treatment practices.
Copper naphthenate is not listed as a RUP by the EPA, nor are there any consumer information
sheets available for guidance on handling and site precautions. However, in Iowa an applicators
license may still be needed for in-place applications on public property. Even though health
concerns do not require copper naphthenate to be a RUP, common sense precautions such as the
use of dust masks and gloves should be followed when handling treated wood.
4.1.3.1. Copper Naphthenate Preservative Field Investigations
The field investigations showed that very few bridge structures have been constructed with
copper naphthenate-treated wood; however their prevalence is becoming more apparent due to
comparable availability, cost, and easier handling. Only relatively new bridge structures were
identified with only flat sawn timber elements. Cap beams, abutment backwalls, stringers,
decking and guard railing were all investigated. Several counties had reservations about using
piles treated with copper naphthenate due to lack of information on water and high moisture area
performance. Although the bridges built with copper naphthenate are still relatively new, the
counties have good feedback on its performance and excellent handling properties. Figures 4-45
through 4-54 show copper naphthenate bridge elements in good condition.
38
Figures 4-45 and 4-46 show copper naphthenate bridge cap beams.
Protection against
nesting animals
Clean surfaces
No Surface
defects
Figure 4-45. Copper naphthenate treated cap beam with building felt cover for protection
from nesting animals
Tear in building felt
cover
Figure 4-46. End of pile cap with building felt cover providing protection from moisture
and weathering
39
Figures 4-47 and 4-48 show copper naphthenate bridge backwalls.
Figure 4-47. Good copper naphthenate backwall
Figure 4-48. Good end grain treatment
40
Figure 4-49 through 4-51 show copper naphthenate bridge stringers.
Figure 4-49. Good copper naphthenate-treated stringers
Clean surface
Good retention
Figure 4-50. Good exterior stringer with no physical defects or excess preservative bleeding
41
Checking
Figure 4-51. Exterior stinger with checking due to in-place seasoning
Figures 4-52 through 4-54 show copper naphthenate bridge deck.
Figure 4-52. Top side of cantilevered copper naphthenate-treated decking
42
Small seasoning
cracks
Figure 4-53. Good end grain of copper naphthenate-treated bridge deck
Underside of
deck
Figure 4-54. Good treatment retention on underside of deck
4.1.3. Oxine Copper
Oxine copper is an inorganic compound consisting of formulations of copper, nickel, and other
inert ingredients. Oxine copper is listed in the AWPA Standards for treating several softwood
species used in exposed, above-ground applications. Oxine copper can be dissolved in a range
of hydrocarbon solvents, but provides longer protection when it is delivered in heavy oil.
Oxine copper solutions are greenish brown, odorless, toxic to both wood decay fungi and insects
and have a low toxicity to humans and animals. Oxine copper solutions are heat sensitive, which
limits the use of heat to increase penetration of the preservative. However, oxine copper has
43
been found to provide good penetration in difficult-to-treat species. Oilborne oxine copper does
not accelerate corrosion of metal fasteners. Oxine copper is not widely used by pressuretreatment facilities.
Wood treated with oxine copper presents fewer toxicity or safety and handling concerns than
other oilborne preservatives. Oxine copper is listed by the U.S. Food and Drug Administration
(FDA) as an indirect additive that can be used in packaging that may come in direct contact with
food. Precautions such as wearing gloves and dust masks should be used when working with
wood treated with oxine copper. Because of its somewhat limited use and low mammalian
toxicity, there has been little research to assess the environmental impact of wood treated with
oxine copper.
No bridges were investigated with oxine copper preservative treatment.
4.2. Waterborne Preservatives
The first waterborne preservatives were developed in the late 1800’s, however; they were not
heavily used until the 1960’s due to a changing demand for clean paintable surfaces (Ritter
1992). Waterborne preservatives are formulations of inorganic arsenical compounds that react
with or precipitate in treated wood. The reaction takes place when members are treated, “fixing”
the precipitants (e.g., copper, chromium, and/or arsenic) within the cells of the wood to help
prevent leaching and migration potential. Waterborne preservatives usually do not cause skin
irritations and are suitable for use where mammalian contact is likely. Thus, waterborne
preservatives are frequently used for guard railings and floors on pedestrian walkways.
Waterborne preservatives are used primarily to treat softwoods, because they may not fully
protect hardwoods from soft-rot attack and because of their micro-distribution within the wood
structure. Waterborne preservatives are also not recommended for large glue-laminated beams
(laminated before treating) because wetting and drying during the treatment process may result
in unwanted dimensional changes, warping, splitting, and cracking. Waterborne preservatives,
however, are used due to their preferred handling properties, clean surfaces, and low leaching
(Wacker 2003).
Waterborne preservative treatments have been found to reduce the mechanical properties of
wood under some conditions. Treatment standards include specific processing requirements
intended to prevent or limit strength reductions resulting from the chemicals and the waterborne
preservative treatment process. The effects of waterborne preservative treatment on mechanical
properties are related to species, mechanical properties, preservative chemistry or type,
preservative retention, post-treatment drying temperature, size and grade of material, product
type, initial kiln drying temperature, incising, and both temperature and moisture in-service.
Waterborne preservatives affect each mechanical property differently with thicker material
undergoing fewer changes than thinner material. Waterborne preservative retention levels of
less than 1.0 lb/ft3 (16 kg/m3) have no effect on modulus of elasticity or compressive strength
parallel to grain and a slight negative effect (-5% to -10%) on tensile or bending strength.
44
Energy-related properties (e.g., load duration and brittle fracture), however, are often reduced
15% to 30%. Air drying after treatment also causes no significant reduction in the static
strength.
4.2.1. Chromated Copper Arsenate (CCA)
Chromated copper arsenate (CCA), often called green treat, was approved for wood use in the
1940’s and has dominated the treatment market from 1970’s until 2004. The EPA no longer
approves the use of CCA for residential construction and has limited its use to certain industrial
and commercial uses which includes timber bridge components.
CCA previously had three standardized formulas: Type A, B, and C, but only CCA Type C
(CCA-C) is still used commercially because it has the best leach resistance and field efficacy.
CCA-C has decades of proven performance and is the reference preservative used to evaluate the
performance of other waterborne wood preservatives. Because of the long usage history, CCA-C
is listed in AWPA standards for a wide range of wood products and applications. CCA-C
protects wood above-ground, in ground contact, or in contact with freshwater or seawater.
Adequate penetration with CCA may be difficult to obtain in some difficult-to-treat species and
is not recommend for hardwood treatments. Chromium inhibits the corrosion of fasteners in
wood treated with CCA more than preservatives that do not include chromium.
CCA contains inorganic arsenic and the EPA classifies it as a RUP. CCA is not available as a
field treatment. Producers of treated wood, in cooperation with the EPA, have created the CIS,
subsequently replaced with the CSIS (Consumer Safety Information Sheet) that gives guidance
on handling and precautions at sites where wood treated with inorganic arsenic are used.
Although CCA has very good handling properties, the CSIS should be available to all persons
who handle wood treated with CCA. For further details an EPA approved CIS can be found in
the Wood Handbook Chapter 14 for inorganic arsenicals (Ibach 1999).
4.2.1.1. Chromated Copper Arsenate Preservative Field Investigations
During the field investigation no bridges were identified that were constructed of CCA treated
wood. The only elements investigated were guard railing and guard rail post. Several of the
railing posts appeared to be reused sign posts. Overall the few CCA treated elements showed
very little decay. Figures 4-54 through 4-57 show performance of CCA-treated guard rail posts
investigated.
45
No visible
decay & good
preservative
penetration
a)
b)
Figure 4-55. a) Guard rail post with mechanical damage and no decay present; b) close up
of damaged area
Preexisting
holes with no
visible decay
Checking
between holes
a)
b)
Figure 4-56. a) Reused CCA guard rail post; b) close up of guard rail post
46
Figure 4-57. Top of a guard rail post showing end grain physical defects
4.2.2. Ammoniacal Copper Zinc Arsenate (ACZA)
Ammoniacal copper zinc arsenate (ACZA) is another common waterborne preservative used for
bridges in the United States. ACZA is a refinement of an earlier formation of ammoniacal copper
arsenate (ACA). ACZA has less arsenic and is less expensive than ACA which has lead to ACA
no longer being available in the United States. ACZA treated wood varies in color from olive to
bluish green. The wood may a have slight ammonia odor that will generally dissipate as the
wood dries.
ACZA contains copper oxide, zinc oxide, and arsenic pentoxide that are dissolved in a solution
of ammonia in water. ACZA has similar performance and characteristics as CCA. However,
ACZA’s chemical composition and stability during treatment at elevated temperatures allows it
to penetrate difficult to treat wood species such as Douglas fir. ACZA is an established
preservative that is used to protect wood from decay and insect attack in a range of exposure
applications in above-ground and ground contact conditions. The ACZA treatment can
accelerate corrosion relative to untreated wood, requiring the use of hot-dipped galvanized or
stainless steel fasteners.
ACZA contains inorganic arsenic and the EPA classifies it as a RUP. ACZA is not available as
a field treatment. Producers of treated wood, in cooperation with the EPA, have created the CIS,
subsequently replaced with the CSIS (Consumer Safety Information Sheet) that gives guidance
on handling and site precautions at sites where wood treated with inorganic arsenic are used.
Although ACZA has very good handling properties, the CSIS should be available to all persons
who handle wood treated with ACZA. For further details an EPA approved CIS can be found in
the Wood Handbook Chapter 14 for inorganic arsenicals (Ibach 1999).
47
4.2.2.1. Ammoniacal Copper Zinc Arsenate Preservative Field Investigations
The use of ACZA was identified in only one county investigated. The oldest ACZA material
inspected were stringers and bridge decking from the early 1990’s. The newest elements
investigated were being placed as backwall plank for a bridge under construction. The county
used ACZA because it was proposed by the supplier, it is an approved IA DOT preservative, and
it has good handling properties for the construction crew. Figure 4-58 through 4-60 show timber
elements treated with ACZA that were investigated.
Plant-treated split
Figure 4-58. New ACZA treated back wall planks properly seasoned prior to treatment
Residual
copper
Checking along
stinger
Figure 4-59. ACZA treated decking and exterior stringer in good condition with only
minor physical defects
48
4.2.3. Alkaline Copper Quaternary (ACQ) Compounds
Alkaline copper quat (ACQ) is one of several wood preservatives that have been developed in
recent years to meet market demands for alternatives to CCA. The fungicides and insecticides in
ACQ are copper oxide and a quaternary ammonium compound. Several variations of ACQ have
been standardized or are being standardized. ACQ type B (ACQ–B) is an ammoniacal copper
formulation that penetrates difficult to treat wood better than other non-ammonical formulations.
ACQ type D (ACQ–D) is an amine copper formulation that provides more uniform surface
appearance and is generally used for treated wood sold at retail lumber yards. ACQ type C
(ACQ–C) is a combined ammoniacal-amine formulation with a slightly different quat compound.
Timber treated with ACQ–B is dark greenish brown and fades to a lighter brown. ACQ-B treated
wood may have a slight ammonia odor until the wood dries. Wood treated with ACQ–D is light
brown and has little noticeable odor. ACQ treatments with these three formulations have
demonstrated their effectiveness against decay fungi and insects in above-ground and ground
contact areas but not in salt water applications (Ibach 1999).
The ACQ formulations are listed in the AWPA standards for a range of applications and many
softwood species. The different formulations of ACQ allow some flexibility in achieving
compatibility with a specific wood species and application. All ACQ treatments accelerate
corrosion of metal fasteners relative to untreated wood. Hot-dipped galvanized or stainless steel
fasteners must be used in structurally critical applications. The number of pressure-treatment
facilities using ACQ is increasing. Researchers at the USDA Forest Service’s Forest Products
Laboratory in Madison, WI are evaluating the performance of a secondary highway bridge
constructed using Southern Pine lumber treated with ACQ–D (Ritter and Duwadi 1998).
Since ACQ does not contain arsenic and has an overall lower toxicity it is not classified as a
RUP by the EPA. Field treatment is possible; however, ACQ is not readily available for field
application purposes. Even though health concerns do not require ACQ to be a RUP, precautions
such as use of dust mask and gloves should be followed when handling treated wood.
4.2.3.1. Alkaline Copper Quaternary Preservative Field Investigations
ACQ is not listed in the Iowa DOT specifications for treatment use, however, it was found to be
used for guard rail repairs. All elements seen were very new and in good condition. Figures 4-60
and 4-61 show the ACQ treated guard rails.
49
ACQ
Railing
Curb &
Railing
Figure 4-60. ACQ treated lumber used as guard railing members
Splitting
Figure 4-61. ACQ railing post with splitting that is generally associated with waterborne
treatments
4.2.4 Other Waterborne Preservatives
Acid copper chromate (ACC) has been used as a wood preservative in Europe and the United
States since the 1920’s. ACC contains copper oxide and chromium trioxide which causes the
treated wood to have a light greenish-brown color and little noticeable odor. The high chromium
content of ACC helps reduce the corrosion of fasteners. ACC does not penetrate difficult-totreat wood species easily and is also more prone to leach than other waterborne treatments (Ibach
1999). The EPA restricts the use of ACC to only non-residential applications, while the AWPA
50
limits its recommended uses to signpost, handrails, guardrails and glue-laminated beams used
above-ground only.
Copper Azole is a recently developed preservative that relies primarily on amine copper and
some additional biocides to protect the member from decay and insect attack. Copper azole type
B (CA-B) is the only formulation currently used in the United States. CA-B contains mostly
copper with some tebuconazole. The treated wood has a greenish brown color with little to no
odor. CA-B is listed by the AWPA for above-ground, ground contact, and critical structure
components. Ammonia can be added to CA-B in order to improve treatment of difficult to treat
wood species. CA-B does increase the rate of corrosion of steel fasteners requiring galvanized,
copper, or stainless steel to be used. CA-B is currently not restricted by the EPA and treatment
plants are becoming more prevalent across North America.
Copper HDO (CX-A) is an amine copper-based preservative that has been used in Europe and
was recently standardized by the AWPA. The active ingredients are copper oxide, boric acid,
and copper-HDO. The appearance and handling characteristics of wood treated with CX-A are
similar to the other copper-based treatments. CX-A formulations have only been standardized
for uses above ground. The availability of CX-A treated material is limited.
4.3. Plant-Applied Preservative Summary
A summary of the discussed plant-applied preservatives is presented in Table 4-1. For
comparison the table includes information on material usage, surface characteristics, color, odor
and fastener corrosion. Not listed in the table are changes in engineering properties, however, as
stated previously oilborne preservatives generally do not reduce engineering properties because
no chemical reaction occurs in the wood’s cellular structure. All waterborne preservatives affect
the engineering properties of the wood and should be accounted for in the design process.
51
Table 4-1. Properties and uses of plant-applied preservatives for timber bridges
Standardized
Uses
Preservative
Solvent
Characteristics
Surface
Characteristics
All uses
Creosote
Oil-type
Oily, not for
frequent human
contact
All uses
Ammoniacal
copper zinc
arsenate
Water
Dry, but
contains arsenic
All uses
Chromated
copper
arsenate
Water
Dry, but use is
restricted by
EPA
All uses
(except in
seawater)
Pentachlorop
henol Type A
(heavy oil)
All uses (except
Copper
in seawater)
naphthenate
All uses
(except in
seawater)
Alkaline
copper quat
All uses
(except in
seawater)
Copper azole
Aboveground,
fully exposed
Pentachlorop
henol Type C
(light oil)
Aboveground,
fully exposed
Aboveground,
fully exposed
Oxine copper
Copper HDO
Color
Odor
Dark brown
Strong,
lasting
Brown,
possible blue
areas
Mild,
short term
Fastener
Corrosion
No worse
than
untreated
Worse than
untreated
wood
Greenish
brown,
weathers to
gray
None
Similar to
untreated
wood
No. 2 fuel oil
Oily, not for
frequent human
contact
Dark brown
Strong,
lasting
No worse
than
untreated
wood
No. 2 fuel oil
Oily, not for
frequent human
contact
Green,
weathers to
brownish
gray
Strong,
lasting
No worse
than
untreated
wood
Water
Dry, okay for
human contact
Greenish
brown,
weathers to
gray
Mild,
short term
Worse than
untreated
wood
Water
Dry, okay for
human contact
Greenish
brown,
weathers to
gray
Mild,
short term
Worse than
untreated
wood
Mineral spirits
Dry, okay for
human contact
if coated
Light brown,
weathers to
gray
Mild,
short term
No worse
than
untreated
wood
Mineral spirits
Dry, okay for
human contact
Greenish
brown,
weathers to
gray
Mild,
short term
No worse
than
untreated
wood
Water
Dry, okay for
human contact
Greenish
brown,
weathers to
gray
Mild,
short term
Worse than
untreated
wood
The longevity or service life of preservative treated wood depends on a range of factors
including type of preservative, treatment quality, construction practices, type of exposure, and
climate. To better understand these factors for long term performance the USDA Forest Service,
Forest Product Laboratory has conducted various field tests since the 1930’s. The most common
of these tests are the stake test that utilizes 2- by 4- by 18 in. Southern Pine sapwood stakes
52
treated with various preservatives and installed in southern Mississippi. The stakes are half
buried in the soil and then periodically removed and inspected for the extent of decay and insect
attack. The stakes were given a rating based on a scale that ranges from 10 (no attack) and 0
(failure, easily broken into pieces). Long term stake performance data were collected for
creosote, penta, ACZA/ACA, and CCA-C. Figure 4-62 shows the average rating of stakes when
treated with ground contact structural critical retentions of each preservative type. The results of
preservatives at this retention level showed CCA-C with the best performance followed by
ACZA/ACA, creosote, and penta. The same trend was found for stakes treated at retentions for
bridge pile usage. No data were available for ACZA/ACA at pile retention values, however,
Figure 4-63 shows CCA-C with the best decay rating relative to creosote and penta.
Figure 4-62. Average ratings of stakes when treated with retentions for structurally critical
structures in ground contact.
53
Figure 4-63. Average ratings of stakes when treated with retentions intended for piling
The superior performance of the waterborne preservatives (CCA and ACZA) in comparison to
creosote and pentachlorophenol in these 2 by 4 stake plots is interesting but may be somewhat
misleading. Unlike the waterborne preservatives, creosote and pentachlorophenol are not
chemically bound to the wood and resist depletion because the oils have low water solubility.
But, the oil-type treatments do very gradually redistribute over time because of the effects of
gravity and expansion and contraction of the wood. In a large member such as a pile or timber
there is substantial reservoir of the oil-type preservative and it takes much longer for the effects
of this redistribution to reduce the loading of preservative below the point of efficacy. In vertical
members such as piles, posts and poles the downward movement from gravity may actually
enhance long term durability by continually replenishing preservative within the wood in the
critical ground-line area. The relatively small stakes used in tests are thus somewhat biased
against the oil-based treatments because they overemphasize the effects of depletion in
comparison to product-size material. The durability of fence posts treated with
pentachlorophenol, creosote and copper naphthenate appears to be greater than that noted for the
stakes (Crawford et al. 2002).
The FPLs comparison of treated posts is expected to be more representative of the performance
of treated piles and poles. For these tests, Southern Pine posts, with diameters of 4 -5 in. were
pressure-treated with preservatives and placed in the ground in southern Mississippi. The posts
were periodically stressed to a possible failure point by the use of a 50 lb (22.73 kg) pull test
(Freeman, et al. 2005). The most recent inspection was conducted after 53 years of exposure, at
which time sufficient posts had failed to allow calculation of expected service life as shown in
54
Table 4-2. The post were treated to retentions below those currently specified in AWPA
standards, however the preservative treatments are performing surprisingly well.
Pentachlorophenol in particular is performing well in these post tests in comparison to the stake
data discussed previously.
Table 4-2. Estimated service life of treated round fence post in southern Mississippi
Average
Retention
(lb/ft3)
Estimated
Service Life
(years)
Copper Naphthenate
0.03
Creosote
Preservative
90% Confidence Limits
for Service Life (years)
Lower
Upper
65
55
78
5.6
54
47
62
Pentachlorophenol
0.32
74
60
91
ACA
0.34
60
51
69
Untreated
0
2.4
2.1
2.7
Determining life expectancy based on the bridges that were field investigated in this project was
quite difficult due to the multitude of variables that cause biodeterioration of different bridge
elements. Comparisons were also difficult because of the small number of bridges constructed
of non-creosote treated timber. The large number of creosote bridges investigated, however, did
reveal general trends for individual bridge elements. Creosote abutment piles that were kept up
and back from the stream channel were found to last 60 to 70 plus years. Creosoted piles located
in the stream channel or in moist areas were generally found to have a have a life expectancy of
40 to 50 years. Creosoted elements that were not in contact with the ground (e.g., stringers)
were generally found to last 50 years or more. The pentachlorophenol and copper naphthenate
bridges were too few and too new to determine any longevity trends from field inspections.
Field investigations also revealed that regardless of treatment type, member protection also
contributed to the longevity and performance of the bridge. Bridge elements that appeared to be
field cut and treated in-place generally had less decay than untreated cut members. Several older
bridges used bituminous coatings on cut or damaged areas helping extend the longevity of the
bridge members. Bridge elements that were protected by the deck, such as interior stringers, had
better performance and less decay relative to members that were exposed. Interior stringers had
very little decay and physical defects, however, the exterior stringers tended to have checking
along the length of the members. When comparing new and old exterior stringers all members
had checking on the face regardless of age. Bridges with wearing surfaces, as seen in Figure 464, were also seen to have less damage and decay than when the deck also was used as the
wearing surface. Although gravel decks can trap and hold moisture, the timber decks with
gravel wearing surfaces were performing better than decks without any added wearing surface.
Bridges without a wearing surface had more mechanical damage and weathering causing decay
and physical defects. The additional damage seen in bridges without a wearing surface will
likely lead to a shortened service life. The overall condition of piles and cap beams that had
metal or felt covers was much better than piles and caps left uncovered. Specifically, a reduction
55
in end grain decay and checking was seen on all piles and caps with covers. Metal and building
felt caps were used for protection, however, metal caps were found to have better longevity and
durability.
Figure 4-64. Good wearing surface protects the timber decking
56
5. IN-PLACE PRESERVATIVES TREATMENTS
For best performance, as much fabrication should be completed prior to pressure treatment to
allow all exposed surfaces to be protected (Duwadi and Ritter 1997). In reality, on-site
fabrication of timber bridge components typically results in breaks in the protective barrier. Pile
tops, which are typically cut to length after installation, specifically need reapplication of the
preservative to the cut ends. Likewise, the exposed end-grain in joints, which is more susceptible
to moisture absorption, and the immediate area around all fasteners, including drill holes, require
supplemental on-site treatment.
Installers should be provided with supplemental preservative and instructions for its safe
handling and proper use during the construction process. Periodic inspections should seek to
identify cracks, splits, and checks that result from normal seasoning as well as areas of high
moisture or exposed end grain in joint areas. These areas require periodic reapplication of
supplemental preservative. Supplemental in-place treatments are available in several forms:
surface-applied chemicals, pastes, diffusible chemicals, and fumigants. Several of the in-place
preservatives are RUP and require certified applicators licensing as was discussed in Chapter 4.
5.1. Surface Treatments
The simplest method for applying supplemental preservative treatment during fabrication or
routine maintenance involves brushing or spraying a preservative onto the known break in the
treatment barrier or over the suspected problem area (e.g., joints, fasteners, pile tops). Flooding
of bolt holes and the tops of cut-off piles are particularly important. Often these surfaces will be
covered or closed during construction and will no longer be available for surface treatment.
Cracks, checks and splits should be retreated during subsequent inspections. Because surface
treatments do not penetrate deeply into the wood where deterioration is mostly likely to occur
and because their application does present some risk to the environment, their use should be
limited to problem areas such as bolt holes, exposed end-grain, checks and splits.
5.1.1. CuNap
For brush or spray applications, copper naphthenate in oil is the preservative that is most often
used. The solution should contain 1 - 2% elemental copper. Copper naphthenate is available as a
concentrate or in a ready-to-use solution in gallon and drum containers.
5.1.2. Borate Solutions
Borate solutions can also be sprayed or brushed into checks or splits. However, because they are
not fixed to the wood they can be leached during subsequent precipitation. Borates are sold
either as concentrated liquids (typically formulated with glycol) or as powders that can be
diluted with water
57
5.2. Pastes
Another type of surface treatment are the water soluble pastes containing combinations of copper
naphthenate, sodium fluoride, copper hydroxide, or borates. The theory with these treatments is
that the diffusible components (i.e., boron or fluoride) will move through the wood; while at the
same time the copper component remains near the surface of a void or check. These pastes are
most commonly used to help protect the ground-line area of poles. After the paste is applied, it
is a covered with a wrap to hold the paste against the pole and prevent loss into the soil. In
bridge piles this type of paste application should be limited to terrestrial piles that will not be
continually or frequently exposed to standing water. These pastes may also be effective if used
under cap beams/covers to protect exposed end-grain. Reapplication schedules will vary based
on the manufacturers recommendations as well as the method and area of application.
5.3. Diffusible Chemicals
Surface-applied treatments often do not penetrate deeply enough to protect the inner portions of
large bridge members. An alternative to surface applied treatments is installation of internal
diffusible chemicals. These diffusible treatments are available in liquid, solid or paste form, and
are applied into treatment holes that are drilled deeply into the wood. They are similar, (and in
some cases identical) to the surface-applied treatments or pastes. Boron is the most common
active ingredient, but fluoride and copper may also be incorporated. In timbers, deep holes are
drilled perpendicular to the upper face on either side of checks. In round piles, steeply sloping
holes are drilled across the grain to maximize the chemical diffusion and minimize the number of
holes needed. The treatment holes are plugged with tight fitting treated wooden plugs or
removable plastic plugs. Plugs with grease fittings are also available so that the paste can be
reapplied without removing the plug.
Solid rod treatments are a good choice in environmentally sensitive areas or in applications
where the treatment hole can only be drilled at an upward angle. However, solid rods may
require more installation effort. Further, the chemical does not diffuse as rapidly or for as great a
distance as compared to a liquid form (De Groot et al. 2000). One reason that the solid forms
may be less mobile is that diffusible treatments need moisture, which is lacking in a solid, to be
able to move through wood. Concentrated liquid borates may also be poured into treatment
holes and are sometimes used in conjunction with the rods to provide an initial supply of
moisture. Fortunately, when the moisture content falls below 30%, little chemical movement
occurs, but growth of decay fungi is also substantially arrested below 30% moisture (Smith and
Williams 1969). Since there is some risk that rods installed in a dry section of a timber would
not diffuse to an adjacent wet section, some experience in proper placement of the treatment
holes is necessary. The diffusible treatments do not move as far in the wood as do fumigants
(described in the subsequent sections), and thus the treatment holes must be spaced more closely.
A study of borate diffusion in timbers of several wood species reported that diffusion along the
grain was generally less than 5 in. and diffusion across the grain was typically less than 2 in. (De
Groot et al. 2000).
Currently, diffusible chemicals are not listed as RUP’s and have the advantages of having
relatively low toxicity and ease of handling. Although many diffusible chemicals list piles for
58
labeled usage, the treatment should be applied so the chemical is deposited above the mean high
water mark on piles.
5.4. Fumigants
Like diffusibles, fumigants are applied in liquid or solid form in predrilled holes. However, they
then volatilize into a gas that moves through the wood. One type of fumigant has been shown to
move over 8 ft from the point of application in poles (Highley and Scheffer 1989). To be most
effective, a fumigant should be applied at locations where it will not leak away or be lost by
diffusion to the atmosphere. When fumigants are applied, the timbers should be inspected
thoroughly to determine an optimal drilling pattern that avoids metal fasteners, seasoning checks,
and severely rotted wood. In vertical members such as piles, holes to receive liquid fumigant
should be drilled at a steep angle (45° to 60°) downward toward the center of the member,
avoiding seasoning checks. The holes should be no more than 4 ft apart and arranged in a spiral
pattern (Highley and Scheffer 1989). With horizontal timbers, the holes can be drilled straight
down or slanted. As a rule, the holes should be extended to within about 2 in. (5.08 cm) of the
bottom of the timber. If strength is not jeopardized, holes can be drilled in a cluster or in pairs to
accommodate the required amount of preservative. If large seasoning checks are present, the
holes should be drilled on each side of the member to provide better distribution. As soon as the
fumigant is injected, the hole should be plugged with a tight-fitting treated wood dowel or
removable plastic plug. For liquid fumigants, sufficient room must remain in the treating hole so
the plug can be driven without squirting the chemical out of the hole. The amount of fumigant
needed and the size and number of treating holes required depends upon the timber size.
Fumigants will eventually diffuse out of the wood, allowing decay fungi to recolonize.
Fortunately, additional fumigant can be applied to the same treatment hole. Fumigant treatments
are generally more toxic and more difficult to handle than the diffusible treatments. Some are
considered to be RUP by the U.S. EPA, requiring extra precautions (Highley 1999) and should
only be applied above the mean high water mark on piles. Another disadvantage of preencapsulated fumigants is the relatively large size of treatment hole required.
5.4.1. Chloropicrin
The most effective fumigant currently used is chloropicrin (trichloronitromethane). Chloropicrin
is a liquid and has been found to remain in wood for up to 20 years; however, 10-year retreatment cycles are recommended with regular inspection (Ritter. 1992). Chloropicrin is a
strong eye irritant and has high volatility. Due to chloropicrin’s hazardous nature it should be
used in areas away from buildings permanently inhabited by humans or animals. During
application workers must wear protective gear including a full face respirator. Advances in
chloropicrin formulations have allowed it to be placed in semi-permeable tubes for slow release.
Using semi-permeable tubes reduces the risks presented to workers if chloropicrin leaks out of
checks and splits in the wood. The tubes further allow for applications above ground were liquid
material would flow out (Morrell et al. 1996).
59
5.4.2. Methylisothiocyanate (MITC)
Methylisothiocyante (MITC) is the active ingredient in several fumigants, but is also available in
a solid-melt form that is 97% active ingredient. The solid-melt MITC is supplied in aluminum
tubes. After the treatment hole is drilled the cap is removed from the tube, and the entire tube is
placed into the whole. This formulation provides ease of handling and application to drilled
treatment holes that slope upward.
5.4.3. Metham Sodium (Vapam)
Metham sodium (sodium N-methldithiocarbamate) is a most widely used fumigant. However,
metham sodium must decompose in the presence of wood in order to create MITC which is the
active fungicide. Metham sodium is not recommended for use in standing water. Metham
sodium is also the least effective fumigant with an estimated protective service life of seven to
10 years in Douglas-Fir timbers. The lower effectiveness is due to lower amounts of active
ingredients after decomposition. Decomposition of metham sodium can be inhibited by wood
species, moisture, and temperature. Metham sodium is also corrosive to fasteners (Morrell et al.
1996).
5.4.4. Granular Dazomet
Dazomet (tetrahydro-3, 5-dimethyl-2-H-1,3,5, thiodazine-6-thione) is applied in a solid granular
form that decomposes to a MITC content of approximately 45%. Dazomet is easy to handle, but
slower to decompose and release MITC than the solid-melt MITC or liquid fumigants. Some
suppliers recommend the addition of a catalyst to speedup the breakdown process.
5.5. In-Place Preservative Summary
A summary of the discussed in-place preservatives is presented in Table 5-1. For comparison
the table includes information on application locations, leaching and diffusing characteristics,
bridge applications, and handling.
60
Table 5-1. Properties and uses of in-place preservatives for timber bridges
In-place
Preservative
Type
Active
Ingredient
Solvent
Type
Internal vs.
External
Leeching or
Diffusing
Bridge Location
Handling &
other
Surface
treatment
liquid
Copper
naphthenate
Oil
External
sprayed or
brushed
Insoluble in
water
Bolt holes, exposed
end grain, checks
& splits
Non-RUP
Surface
treatment
liquid or
powder
Borate
solutions
Water
External
sprayed or
brushed
Leach away by
precipitation
Bolt holes, exposed
end grain, checks
& splits
Non-RUP
Surface
treatment
paste
CuNap,
sodium
fluoride, CuHydrooxide,
borates
Water
External &
covered with
wrap
Boron &
fluoride move
into wood,
Copper stays
at surface
Ground line area of
terrestrial piles &
under pile caps
Non-RUP
Diffusible
Chemical
Liquid
Boron,
fluoride,
copper
Water
Internal
through
drilled holes
Needs
moisture to
diffuse into
wood
Pile & deep
timbers w/ drill
accessibility
Non-RUP,
Low toxicity
& ease of
handling
Fumigant
liquid
Chloropicrin
NA
Internal
through
drilled holes
Volatizes into
gas & move
into wood
Pile & deep
timbers w/ drill
accessibility
RUP
Fumigant
Solid
Solid-melt
MITC
NA
Internal
through
drilled holes
Volatizes into
gas & move
into wood
Pile & deep
timbers w/ drill
accessibility
RUP
Fumigant
liquid
Methan
Sodium
(Vapam)
NA
Internal
through
drilled holes
Volatizes into
gas & move
into wood
Pile & deep
timbers w/ drill
accessibility
RUP
Fumigant
Solid
Granular
Dazomet
NA
Internal
through
drilled holes
Volatizes into
gas & move
into wood
Pile & deep
timbers w/ drill
accessibility
RUP
NA = Not Applicable
61
6. INSPECTION TOOLS AND TESTING
A number of tools exist to assist the inspector with the diagnosis of deterioration and
preventative maintenance. The tools vary considerably in the amount of experience required for
reliable interpretation, accuracy in pin-pointing a problem, ease of use, and cost. No single test
should be relied upon for inspection of timber bridge components. Rather, a standard set of tools
should be used by inspectors to ensure conformity in inspections and uniformity between
inspectors.
6.1. Visual Assessment
A general visual inspection can give a quick qualitative assessment for corroded fasteners, split,
cracked, and checked wood; and crumbling, collapsed, fuzzy, or discolored wood. All color
changes in the wood, such as darkening, presence of bleaching, staining, and signs of moisture
accumulation in a joint or on any wood surface should be noted. Wood with advanced brown-rot
decay turns dark brown and crumbly with a cubical appearance or may be collapsed from
structural failure. White-rot decay is characterized by bleaching and the wood appears whiter
than normal. White-rotted wood does not crack across the grain like brown-rotted wood and
retains its outward shape and dimensions until it is severely degraded. Soft rot decay is most
likely to occur at the water line. Soft rot is characterized by a shallow zone of decay on the
wood surface that is soft to the touch when the wood is wet, but firm immediately beneath the
surface. Staining of the wood can be caused by mold or stain fungi, watermarks or rust stains
from metal fasteners. Stain generally points to areas that have been wet or where water has been
trapped. Salt abrasion, from spills or splashes gives wood a fuzzy appearance and is primarily a
concern because it can damage the protective barrier of the preservative.
Listed below are definitions of several physical properties and defects that can be visually seen
as indications of protective performance or may suggest areas of future concern.
•
•
•
•
•
•
•
Checks: Longitudinal separations that extend perpendicular to the growth rings at the
end grain of a member.
Decay at Fasteners: Biodeterioration at holes and cuts used to connect bridge
members together.
End Grain Decay: Biodeterioration at the ends of board or other timber members that
extend into the member parallel to the grain
Splitting: Damage at the end grain of a log or board that extends perpendicular
through the board from face to adjacent face.
Stain: Discoloration on the wood surface
Surface Decay: Biodeterioration on the exterior faces of a timber member
Ultraviolet degradation: Chemical reactions causing a grayish color of wood that is
easily eroded from the surface exposing new wood cells; also called weathering.
Additional defects that may pose structural or aesthetic concerns but are not necessarily an
indicator of preservative performance include:
62
•
•
•
•
•
Bow: Curving or arching of a boards length perpendicular to the flat face or depth of
the member.
Crook: Curving or arching of a boards length perpendicular to the edge or breadth of
the member
Cup: Curving or arching of a board across its depth so the board no longer lays flat.
Shake: A separation or plane of weakness between and paralleling the growth rings
extending in the longitudinal direction of the member.
Twist: Warping of a board about the longitudinal or length wise axis so that the four
corners of the board no longer rest in the same plane.
6.2. Probing & Pick Test
Use of an awl or other sharp pointed tool can detect soft spots created by decay fungi or insect
damage. Probing can locate pockets of decay near the surface of the wood member or can be
used to test the splinter pattern of a piece of wood. Non-decayed wood is dense and difficult to
penetrate with the probe and results in a fibrous or splintering break (Wilcox 1983). In a fibrous
break, splinters are long and separate from the wood surface far from the tool. A splintering
break results in numerous splinters directly over the tool. A pick test on non-decayed wood will
give an audible sound that one would expect to hear when wood breaks. A pick test on decayed
wood will result in a brash or brittle failure across the grain with few, if any, splinters. The
sound will not be as loud. The pick test can subjectively differentiate between sound and
decayed wood in weathered specimens that might otherwise be mistaken as decayed under
comparable conditions. This simple test does require some experience to reliably interpret the
results.
6.3. Moisture Measurement
Moisture measurements are taken with an electronic hand-held moisture meter. The moisture
meter consists of two metal pins that are pushed into the wood. The meter displays a
measurement of electrical resistance (moisture content) between the pins. Moisture content
greater than 20% indicates that enough moisture is present for decay to begin. Moisture
measurements provide information on areas where water is being trapped, such as joints, and
serves as an indicator that a more thorough assessment of an area with high moisture content is
necessary.
6.4. Sounding
In this method, a hammer is used to strike the wood surface. Based on the tone, the inspector
must be able to differentiate a hollow sound created by a void or pocket of decay from the tone
created by striking sound wood. Some experience is necessary for reliable interpretation of
sounding since many conditions can contribute to variations in sound quality. Sounding method
is best used in conjunction with other inspection methods (Ross et al. 1999).
63
6.5. Stress Wave Devices
Stress wave devices measure the speed (transmission time) at which stress waves travel through
a wood member. Stress wave measurements locate voids in wood caused by insects, decay fungi
or other physical defect. Stress wave signals are slowed significantly in areas containing
deterioration. Because stress wave signals do not distinguish between active decay, voids, ring
shakes or other defects, this method should be used with other inspection methods (Clausen et al.
2001).
6.6. Drill Resistance Devices
Drill resistance devices record the resistance required to drill through a piece of wood. The
amount of resistance is related to the density of the wood in that particular area and can be used
to determine if deterioration exists. This method should be used with other inspection tools
(Emerson et al.1998).
6.7. Core Boring
Increment core borings of representative areas should be taken perpendicular to the face of the
member being sampled. All test holes must be plugged immediately after extracting the
increment core with a tight-fitting wood plug treated with a preservative similar in performance
to the member being sampled. Increment cores can be visually examined for signs of
deterioration and may be submitted to a laboratory for biological and/or chemical analysis.
6.8. Preservative Retention Analysis
In most cases the pressure-treated shell in bridge members contains more than enough
preservative to protect the wood. However, in older members, or in situations where
deterioration is evident in the treated shell, analysis may be a worthwhile means to determine the
preservative retention characteristics. Preservative retention can be determined from a wood
sample by an analytical chemist using AWPA standardized test methods. A list of recognized
methods (A15-03) is provided by AWPA to assist in the determination of preservative retention
in freshly treated or aged wood. Instrumentation necessary for analysis and associated methods
vary for each preservative treatment. Recommended methods of analysis for preservative
treatments commonly used in timber bridge construction during the past 10 years are provided
and referenced here.
Creosote
AWPA standard A6-01 (AWPA 2007) is specified for the determination of oil-type preservatives
in wood. Wood borings or samples that have been reduced to shavings, chips or slivers are
extracted with toluene to provide a qualitative analysis of residual creosote in aged wood. The
volume of wood extracted (i.e. diameter of the drill bit for drill shavings) must be known to
calculate retention on a lb/ft3 or kg/m3 basis.
64
Pentachlorophenol
The Volhard Chloride procedure, commonly referred to as “lime ignition”, is one method of
analysis of wood treated with pentachlorophenol. An alternative method, the copper pyridine
method, can be used for the determination of technical pentachlorophenol and should be used
when a method that is specific for chlorinated phenols is required. Both methods are described in
AWPA standard A5-05 (AWPA 2007).
Copper naphthenate
The method for chemical analysis of wood treated with copper naphthenate (A5-05) is based on
the oxidation of iodide to iodine by cupric ions followed by titration of iodine by thiosulfate. The
method essentially determines the total copper in a sample. Results are expressed as copper
metal (AWPA 2007).
Metallic elemental analysis
Elemental copper, chromium, arsenic, zinc and boron can be determined by inductively coupled
plasma (ICP) emission spectrometric analysis for any of the following preservatives: CCA,
ACC, ACZA. The test is conducted following AWPA standard A21-00 (AWPA 2007).
Elemental determination in ppm (parts per million) should be converted to and reported in the
oxide form of the metal. Metallic elemental analysis will be used for ACQ and CA-B
determinations in the future for new installations. Copper, chromium, arsenic and zinc
concentrations in treated wood can also be determined using X-ray spectroscopy as described in
AWPA standard A9-01.
65
7. SPECIFICATIONS AND GUIDELINES
7.1. Iowa Department of Transportation State Specification
State of Iowa specifications pertaining to the handling and preservative treatment of timber used
for bridges can be found in the Iowa Department of Transportation (Iowa DOT) Standard
Specifications with GS-01013 revision. The Iowa DOT also has requirements for quality control
protocol, approved treated timber suppliers, and treatment plants in Materials Instructional
Memorandum Volume 1 Section 462. The specifications and memorandum can be found online
at http://www.erl.dot.state.ia.us/.
Division 41 of the standard specifications contains specific information on wood preservatives
and preservative treatments. Listed in this division are the AWPA chemical requirements for
creosote, penta type A, copper naphthenate type A, CCA type A, B, and C, and ACZA. The five
specific preservatives listed are required to be used unless no funding from state or federal
sources are used.
The state specification requires preservative retentions to be in accordance with the
recommendation of AWPA Standard U1 and the applicable AWPA Commodity specifications as
shown in Table 7-1. The minimum preservative penetration required for Douglas-Fir and
Southern Pine are also shown in Table 7-2 for different uses. The penetration requirements are
based on AWPA standards.
Table 7-1. Minimum preservative retention requirements
Retention (lb/ft3)
Material and Usage
Creosote(2)
Pentachlorophenol(2)
Copper
Naphthenate(2)
ACZA(3)
CCA(1,3)
AWPA
UC-SectionSpecial Req.
Lumber and Timber for
AWPA
AWPA
AWPA U1
AWPA U1
AWPA U1
AWPA U1
Structures
U1
U1
Piles for Foundation:
UC4C-E
Douglas Fir
17
NR
0.14
NR
NR
Southern Pine
12
NR
0.1
NR
NR
Post, Guardrail, and
Spacer Blocks:
Sawed Four Sides
NR
0.6
0.075
0.5
0.5
UC4A-B
Posts, Fence, Guide,
and Sign:
Round
NR
0.4
0.055
0.4
0.4
UC4A-B
Sawed Four Sides
NR
0.5
0.06
0.4
0.4
UC4A-A-4.3
Note: (1) CCA shall not be used for treatment of Douglas Fir
(2) Oil type preservatives
(3) CCA, ACA, and ACZA are waterborne preservatives
(4) Retentions based on AWPA. Use Category and Commodity Specification for different applications
NR = Not Recommended
66
Table 7-2. Minimum preservative penetration requirements
Penetration (in and/or % of sapwood penetration)(1)
Southern Pine
Douglas-Fir
AWPA Material Standard
Section
Lumber and Timber for
Structures
AWPA U1 T1
AWPA U1 T1
AWPA U1 T1
Piles for Foundation
2.5 in. or 85%
0.75 in. and 85%
up to 1.6 in. and 85%
T1-8.5
2.5 in. or 85%
Under 5 in. thick:
0.4 in. and 90%
5 in. and thicker
0.5 in. and 90%
T1-8.1
2.0 in. or 85%
3/8 in. and 100%
up to 1 in. or 85%
T1-8.2
2.5 in. or 85%
Under 5 in. thick:
0.4 in. and 90%
5 in. and thicker
0.5 in. and 90%
T1-8.1
Material and Usage
Post, Guardrail, and Spacer
Blocks:
Sawed Four Sides
Posts, Fence, Guide, and Sign:
Round
Sawed Four Sides
Note:
(1) Penetrations based on AWPA. Use Category and Commodity Specification for different applications
Other requirements for the treatment process found in Division 41 include the following
information.
•
•
•
•
•
•
•
•
Coastal Douglas-Fir shall be incised.
Waterborne treated material is required to be seasoned prior to and after treatment.
The moisture content requirements pre and post-treatment are 20% and 23% for kiln
dried and air dried material, respectively. The moisture content shall be determined
using a resistance type moisture meter.
To avoid oil accumulation on guardrails and sign posts, the specification requires a
steam and vacuum process prior to removal from the treatment cylinder.
The full cell treatment process is to be used for waterborne preservatives and the
empty cell process with initial air pressure is required for oil preservatives.
The results of the treatments are to conform with Tables 7-1 and 7-2 and AWPA
Standards U1 and T1. The retentions are to be determined by analysis methods.
Handling of the product after treatment is to be in accordance with AWPA Standard
M4 and the individual pieces are to be marked with the appropriate identification
brand, stamp, or tag.
All inspection certifications and test reports for each shipment are to be provided
according to Iowa DOT specification Material I.M. 462.
Only Douglas-Fir (costal region), Northern Pine, and Southern Pine are allowed to be
treated. The structural members must also be pre bored and cut prior to treatment
whenever possible.
67
Division 24 of the specifications describes construction practices and in-place treatment of cut
timber members. The cut surfaces of pile heads must be treated with copper naphthenate.
Division 24 also states all newly exposed surfaces (e.g., in-field framing and boring) shall be
coated with two coats of copper naphthenate.
The Materials Instructional Memorandum states that treated timber products used in timber
structures must be supplied by approved suppliers and treatment plants. If any timber material is
furnished by an unapproved source, the material shall not be accepted. The steps for becoming
an approved supplier and treatment plant are listed within the memorandum. Included in the
memorandum is information pertaining to plant treatment quality control, material handling, and
criteria for material identification. The appendices for the memorandum contain pre approved
treatment plants and suppliers.
7.2. American Wood Protection Association Standards (AWPA)
The American Wood Protection Association (AWPA) is the primary standard-setting body for
preservative treatment in the United States. The AWPA Standard-07 contain standards for Use
Category System (UCS) Standards, Nonpressure Standards, Preservative Standards, Analysis
Method Standards, Miscellaneous Standards, and Evaluation Standards. The UCS standards and
Miscellaneous standards are the most applicable to timber bridge preservatives. UCS standards
also identify proper preservative retention and penetration for various timber materials. The
Miscellaneous Standards have sections pertaining to the care of preservative treated wood and
guidelines for pole maintenance programs. These programs may possibly be adapted to bridges.
To guide selection of the types of preservatives and loadings appropriate to a specific end-use,
the AWPA recently developed the UCS standards (AWPA 2007). The UCS standards simplify
the process of finding appropriate preservatives for specific end-uses. AWPA groups treated
wood applications by the service environment and the timber usage. The service environment is
divided further by use category designations. The AWPA has five use categories with the lowest
category, UC1, for wood that is used in interior construction and kept dry; while the highest,
UC5, includes applications that place treated wood in contact with seawater and marine borers.
The use category designations also integrate the structural importance of members. Most
applications for highway construction fall into categories UC4B and UC4C.
To specify the proper treatment and penetration of different bridge elements the use category
designations are used in conjunction with the Commodity Specifications (U1) and the Processing
Standards section (T1) of the UCS. The Commodity Specifications have nine classifications
(Section A through I) for relating appropriate preservative retentions and the member usage. The
Processing Standard, Sections 8.1 through 8.9, provide penetration requirements appropriate to
species and use categories. To use the UCS standards the intended use category and the
commodity classification must be known. Table 7-3 shows the use category, Commodity
Specifications, and Processing Standard for most timber bridge elements.
68
Table 7-3. AWPA Use Category and Commodity Specifications for timber bridge elements
Bridge
Element
Commodity
Use
Exposure
Use
Category
Commodity
Specification (U1)
Section
Special
Reqs
Processing
Standards
(T1)
Piling
Piles, round
Highway
construction
Ground contact
or fresh water
4C
E
-
8.5
Backwall
Lumber &
timbers
Highway
construction
Ground contact
or fresh water
4B
A
4.3
8.1
Cap beam
Lumber &
timbers
Highway
construction
Ground contact
or fresh water
4B
A
4.3
8.1
Stringer
Lumber &
timbers
Highway
construction
Ground contact
or fresh water
4B
A
4.3
8.1
Decking
Decking
Highway
bridge
structural
Above ground
4B
A
4.3
8.1
Gluelaminated
beams and
panels
Gluelaminated
beams
Highway
important
structural
Ground contact
or fresh water
4B
F
-
8.6
Gluelaminated
beams and
panels
Gluelaminated
beams
Highway
critical
structural
Ground contact
or fresh water
4C
F
-
8.6
Handrails
&
guardrails
Handrails &
guardrails
Highway
construction
Above ground,
exterior
3B
A
4.3
8.1
Post round
Highway
construction
including
guide, sign
and sight
Ground contact
or fresh water
4A
B
-
8.2
Guardrail
post &
spacer
block
Post round
Highway
construction
including
guardrail
posts, spacer
blocks
Ground contact
or fresh water,
moderate decay
4B
B
-
8.2
Guardrail
post & sign
post
Post (sawn 4
sides)
Highway
construction,
general
Ground contact
or fresh water
4A
A
4.3
8.1
Guide,
Sign, &
Site Post
The AWPA Standard for the Care of Preservative-Treated Wood Products (Standard M4)
describes requirements for the care of treated piles and lumber at storage yards and on job sites.
The standard state that all boring, framing, chamfering, etc. should be done prior to treatment
whenever practical. If fabrication must be done in the field, however, surface treatment shall be
applied to areas where the preservative barrier has been broken. Copper naphthenate is
69
recommended in the standards for most field applications; however, coal tar roofing cement can
also be used for patching nail holes, bolt holes and other damaged areas. Timber piles, in
addition to surface treatments, are required to have galvanized metal or aluminum sheets
securely fastened to their tops for end grain protection. In addition to in-place treatment of
members, reuse, burning, and disposal practices are outlined within the standard.
The AWPA also has guidelines for a pole maintenance program. Although the information is
presented for utility and pole owners the same maintenance principals may be able to be applied
to bridges. The guidelines discuss various components for an effective maintenance program.
The first requirement is to have properly trained personnel and a quality control process to insure
that trained personnel, whether in-house or a consultant, perform the work as specified. The next
major requirement is to perform routine inspections. The inspection methods described in
Chapter 6 are the same inspection tools presented in the guidelines. However, partial and full
excavation techniques are additional steps outline that help to ensure decay is not forming below
the surface. After inspections have taken place, evaluation of the structural integrity must be
determined as well as the in-place maintenance or remaining service life. In-place treatments,
discussed in Chapter 5, are suggested for remedial treatment. Lastly, bridge marking, record
keeping, and data management are indicated to be vital for a successful maintenance program.
Good records can help identify changes to new or in-place details.
7.3. American Institute of Timber Construction (AITC)
The American Institute of Timber Construction (AITC) has a Standard for Preservative
Treatment of Structural Glued-Laminated Timber (AITC 109-2007). AITC 109-2007
incorporates the AWPA Use Category System for the treatment of glued-laminated timber
members. AITC suggests, however, that exterior bridge structural members not in direct contact
be classified as use category UC3; AWPA suggests that important highway structural elements
in high decay locations have use category UC4.
The AITC standard also has design considerations that should be considered when selecting the
proper preservative treatment. One design consideration is whether glued-laminated timber
should be manufactured with lumber treated prior to gluing or after gluing. Southern Pine is
generally the only species available for pre-gluing treatment. The preservatives that can be used
for pre-gluing treatment are limited to pentachlorophenol Type C and waterborne treatments.
However, the standards do not recommend waterborne treatments pre-or post-lamination due to
dimensional changes, warping, checking and splitting that can occur with waterborne treatments.
The treating facility limitations must also be considered when designing large glued-laminated
members.
As with the Iowa DOT specifications and the AWPA standards, the AITC also suggests that all
fabrication and machining should take place prior to treatment. AITC references the AWPA
standards M4 for care after treatment and field treatments of glued-laminated timber members.
70
7.4. Best Management Practices for Use of Treated Wood in Aquatic Environments
Due to the increased concerns for the aquatic ecosystems where treated wood bridges and
walkways are placed, the Best Management Practices (BMP) have been developed as a guideline
to reduce their impact on the environment. Much of the BMP are dedicated to the plant-applied
treating process of timber. Through proper treatment selection, good housekeeping practices,
and appropriate post treatment practices, the risk of biological impact on the environment is
greatly reduced before the timber elements arrive on site. However, the BMP also include
guidelines for the construction and maintenance of these structures in order to reduce biological
risk
BMP are a combined effort of all parties involved with the construction of timber bridges. The
treatment producer, designer, owner, and contractor all have important roles in ensuring a clean
environment at a bridge location. During the design process, details should be developed to
reduce field cutting; which allows for better preservative treatment and reduces the amount of
breaks in the protective treatment barrier. Making the contactor aware of BMP during the
bidding process is also important and allows the contractor to plan and budget properly (WWPI
2006).
To ensure minimal contamination of the aquatic environment, all materials should be inspected
upon delivery. The surfaces of the members should be free of loose debris and excess surface
chemicals. If members are found to have areas of concern they should be placed in areas of low
susceptibility to debris transfer or rejected altogether. After the elements have been inspected
they should be stored off the ground in a well drained area away from standing water. If the
material is to be stored for an extended period of time the members should be protected from
precipitation (WWPI 2006).
Field cutting and fabrication should be done away from water and sensitive areas to eliminate
direct infiltration of saw dust and shavings. The timber waste, including sawdust and shavings,
should be collected and properly disposed of. The easiest way to do this is to create a cutting
station were members can be carried for fabrication and field treatment. At the cutting station
tarps can be place on the ground to facilitate easy debris collection. If the members to be cut are
already incorporated into the structure and cannot be removed, tarps may be spread under that
part of the structure before cutting. The use of tarps to contain sawdust becomes more difficult
in windy or rainy conditions. Shavings from drilling holes are generally easier to contain in a
small area than is sawdust. Plastic tubs are useful collection devices when drilling holes on-site.
The importance of this collection work should be stressed in planning and budgeting for the
project so that the construction crew clearly understands that debris collection is an integral part
of the construction process (WWPI 2006).
Even wood properly treated with oil-type preservatives may create an oily sheen when it initially
contacts standing water. This sheen is generally aesthetic and will dissipate and breakdown in a
short time. However, the excess oil can be contained and collected by floating an absorbent
boom around or downstream from the structure. An absorbent boom will also help to contain
any accidental spillage of field-treatment preservative during construction. Having absorbent
pads on hand at the construction site is also a good practice in case members were not adequately
71
conditioned and begin to bleed preservative. Any absorbent materials that have been used for
collection of preservative must be disposed of using appropriate procedures (WWPI 2006).
Any untreated wood that is exposed during field fabrication should be treated to prevent decay.
However, like the treated wood itself, these field treatment preservatives contain ingredients that
could be toxic to aquatic organisms. Field treatment preservatives should be applied sparingly
and with care to avoid spillage. Whenever possible, the field treatment should be applied to the
member before it is placed in a structure over water. Excess preservative should be wiped from
the wood. If the preservative must be applied to wood above water, a tray, bucket, pan or other
collection device should be used to contain spills and drips. Field treatments should not be
applied in the rain to wood that is above water (WWPI 2006).
A free down load of BMP for the use of treated wood in aquatic and other sensitive
environments can be found at www.wwpinstitute.org or a modified version for the state of
Michigan can be found at www.fs.fed.us/na/wit (WWPI 2006; Pilon 2002).
7.5. Specification Summary
If state or federal funding is used for any bridge element made of timber, the Iowa DOT
specifications are the governing body for preservative treatment. Even if no state or federal
funding is used, the Iowa DOT specification and Instructional Memorandum are still
recommended for retention, penetration, and certifications for timber treatments. The current
Iowa DOT specifications are based on AWPA 2006 standards; however, some differences and
clarifications are noted as follows:
•
•
•
•
•
Iowa DOT specifies that lumber and timber shall be treated to current AWPA U1 and
T1 standards for retention and penetrations, respectively. By specifying AWPA
treatment standards, all lumber and timber uses can be encompassed by the use
category system. Table 7-3 can be used for specifying proper use category,
commodity specifications, and processing standards for lumber and timber.
Although not specifically stated, the Iowa DOT classifies glued-laminated beams and
panels as lumber and timber. Hence, Table 7-3 can be used to determine the proper
treatment categories for glued-laminated materials.
Iowa DOT allows piles to be treated with only creosote or copper naphthenate and
has certified treatment plants and suppliers for piles treated with only these two
treatments.
Penetration standards differ slightly between current Iowa DOT specification and
AWPA standards. Both standards list values for foundation piles, however only
AWPA lists values for land and freshwater piles. The pile penetration levels required
by AWPA and Iowa DOT are shown in Table 7-4. The higher penetration values
provide by AWPA for land and fresh water piles are recommended for timber piles
used for bridges.
Iowa DOT does not allow creosote for post, guardrail elements, and spacer blocks.
The restricted use of creosote for these elements is due to the higher probability of
human contact.
72
•
•
•
As shown in Table 7-5, the Iowa DOT has higher retentions than required by AWPA
for guardrail post and spacer blocks. The higher retention levels are based on past
specified requirements and good performance of in-place guardrail members.
Round guardrail posts are not allowed by the Iowa DOT specifications, therefore, no
treatment values are listed.
Although the Iowa DOT has ACZA retention and penetration levels listed within
their specification there are currently no approved certified treatment plants for
ACZA. If ACZA is to be used, an ACZA plant will have to be certified by the Iowa
DOT.
Table 7-4. AWPA and Iowa DOT specification preservative penetration requirements
Penetration (in and/or % of sapwood penetration)
Material and Usage
Southern Pine
Douglas Fir
AWPA Material Standard
Section
2.5 in. or 85%*
0.75 in. or 85%
up to 1.6 in. and 85%*
T1-8.5*
2.5 in. or 85%**
0.75 in. and 85%
up to 1.6 in. and 85%**
T1-8.5**
NS*
NS*
NS*
3.0 in. or 90%**
0.75 in. and 85%
up to 1.6 in. and 85%**
T1-8.5**
Piles for Foundation:
Foundation Piles (entirely
embedded in ground)
Land and Fresh Water Piles
(entirely or partially
embedded in soil or water)
Note:
NS = Not Specified
* = Iowa DOT Specification Requirements
** = AWPA Standards 2007 Requirements
Table 7-5. AWPA and Iowa DOT specifications preservative retention requirements
Retention (lb/ft3)
Creosote
Pentachlorophenal
Copper
Naphthenate
ACZA
CCA
Douglas Fir
17*
17**
NR*
0.85**
0.14*
0.14**
NR*
1**
NR*
NR**
Southern Pine
12*
12**
NR*
0.6**
0.1*
0.1**
NR*
0.8**
NR*
0.8**
NR*
10**
0.6*
0.5**
0.075*
0.060**
0.5*
0.4**
0.5*
0.4**
Material and Usage
AWPA
UC-SectionSpecial Req.
Piles for Foundation:
UC4C-E*
UC4C-E**
Guardrail Post, and Spacer
Blocks:
Sawed Four Sides
Note:
NR = Not Recommended
* = Iowa DOT Specification Requirements
** = AWPA Standards 2007 Requirements
73
-*
UC4A-A**
8. SUMMARY, CONCLUSIONS AND RECOMMENDATIONS
The objective of this study was to evaluate the performance of different wood preservatives in
the field and to review specifications and testing procedures to provide an adequate level of
timber treatment. To complete these objectives, the BEC in conjunction with the FPL evaluated
various types of preservatives available and reviewed current preservative specifications.
In order to obtain comprehensive conclusions regarding both plant-applied and in-place
treatments, several variables were evaluated including preservative type, age, exposure condition
bridge element location, engineering properties, and environmental issues. The evaluation also
included field investigations of 47 bridges located in various counties in Iowa. The
investigations involved visual inspections of all available bridge elements for decay, physical
defects, and damage.
Decay severity, preservative penetration, and retentions levels can be evaluated using the
inspections tools and testing procedures outlined in this study. The tools and procedures
included destructive, nondestructive, and chemical analysis techniques.
The specifications reviewed included Iowa DOT, AWPA, and AITC. In general, the AWPA was
found to be the primary standard-setting body for preservative treatments and is the basis for the
other specifications reviewed.
Based on the evaluated preservative information, field observations, and review of specifications
and testing procedures the conclusions related to timber bridge preservative performance are:
1. Copper naphthenate is recommended as the plant-applied preservative treatment for
timber bridge elements. Copper naphthenate has been tested extensively by the FPL
in past years and has shown that it has comparable, if not better, performance to other
commonly used preservatives such as creosote. Additional reasons for
recommending copper naphthenate include good handling characteristics, clean
surfaces, comparable availability to other preservatives, and the potential for less
environmental impacts.
2. During the construction of timber bridges, the Best Management Practices should be
followed to minimize environmental impacts to the surrounding ecosystem and
ensure quality treatment of both plant-applied and in-place preservatives. In addition
to the best management practices, bridge owners need to insure pile tops and cap
beams are protected from moisture by use of metal covers and all field cuts are
treated with in-place treatments.
3. The AWPA standards are the basis for the Iowa DOT specifications that are the
regulating standards for bridges being constructed with state or federal funding in the
state of Iowa. If the bridges are being constructed without state or federal the Iowa
DOT specifications and plant certifications are still recommended.
4. Treated Southern Pine piles are recommended to have penetration of 3.0 in. or 90%
of sapwood penetration. The penetration is in accordance with AWPA standards and
is currently stricter than Iowa DOT specifications.
74
5. Timber bridge maintenance programs need to be developed and implemented. A
maintenance program that utilizes combinations of inspection tools and various inplace treatments can easily extend a bridge’s service life. Future work could entail
development of a timber bridge maintenance program for bridge owners. An
effective maintenance program contains many components that need developed
including the following:
a. Personnel training and education: This would include quality control
procedures workers must follow in order to insure work is performed
properly.
b. Inspection procedures: This would include a step-by-step illustrated
guide for inspections
c. Evaluation of structure and restoration: This includes procedures for
evaluating structural condition and developing systems to strengthen
deteriorated areas similar to techniques presented in Vol. 2 of White et
al. 2007.
d. In-place treatment: Similar to inspection procedures this would include a
step-by-step guide for various in-place treatments
e. Records and data management: This includes development of a
searchable database to allow owners to query records for determining
inspections frequency and problem elements.
6. Future workshops and/or short courses presenting biodeterioration and preservative
concepts to timber bridge owners, designers, and inspectors are recommended in
order to implement the information, procedures, etc. presented in this study.
75
CITED REFERENCES
American Society of State Highway and Transportation Officials. 1983. Manual for maintenance
and inspection of bridges. Washington, DC: ASHTO.
AWPA. 2007. Book of Standards. Birmingham, AL: American Wood Preservers Association.
Brient, J. A., Freeman, M. H. and McIntyre, C. R., 2004. Copper Naphthenate Update.
Proceedings for the 100th American Wood Preservers Association 100th Annual
Meeting, Vancouver, British Columbia.
Clausen, C.A.; Ross, R.J.; Forsman, J.W.; Balachowski, J.D. 2001.Condition assessment of roof
trusses of Quincy Mine Blacksmith Shop in Keweenaw National Historical Park. Res.
Pap. FPL-RN-0281. Madison, WI; USDA, Forest Service, Forest Products Laboratory.
Crawford, D.M., DeGroot, R.C., Gjovik, L. R. 1999. Ten-year performance of treated
northeastern softwoods in aboveground and ground-contact exposures. Res. Pap FPLRP-578. Madison, WI: USDA, Forest Service, Forest Products Laboratory.
Crawford, D.M., Woodward, B.M., and C.A. Hatfield. 2002. Comparison of wood preservatives
in stake tests- 2000 Progress Report. Res. Note FPL-RN-02. Madison, WI: USDA,
Forest Service, Forest Products Laboratory.
De Groot, Rodney C.; Felton, Colin C.; Crawford, Douglas M. 2000. Distribution of borates
around point source injections in wood members exposed outside. Res. Note FPL–RN–
0275. Madison, WI: USDA, Forest Service, Forest Products Laboratory.
Duwadi, S.R. and Ritter, M.A. 1997. Timber bridges in the United States. Public Raods on-line
(Winter 1997) Vol. 60, No. 3. www.tfhrc.gov/pubrds/winter97/p97wi32.htm
Emerson, R.N.; Pollock, D.G.; Kainz, J.A.; Fridley, K.J.; McLean, D.L.; Ross, R.J. 1998.
Nondestructive evaluation techniques for timber bridges. In for Natterer, J.; Sandoz, J.-L.
ed(s). Proceedings for the 5th World Conference on Timber Engineering. Montreaux,
Switzerland.
Freeman, M.H., Crawford, D.M., Lebow, P.K. and J. A. Brient. 2005. A comparison of wood
preservatives in posts in southern Mississippi: Results from a half-decade of testing. In:
Proceedings for the American Wood Preservers’ Association.101st. New Orleans,
Louisiana.
Highley, T.L. 1999. Ch. 13. Biodeterioration of Wood. In Wood handbook-Wood as an
Engineering Material. Gen Tech. Rep. FPL-GTR-113. Madison, WI: USDA, Forest
Service, Forest Products Laboratory.
Highley, T.L. and Scheffer, T. 1989. Controlling decay in waterfront structures: evaluation,
prevention and remedial treatments. Res. Pap. FPL-RP-494. WI.: USDA, Forest Service,
Forest Products Laboratory.
76
Ibach, R. E. 1999. Ch. 14, Wood Preservation. In Wood handbook-Wood as an engineering
material Gen. Tech. Rep. FPL–GTR–113: Madison, WI: USDA, Forest Service, Forest
Products Laboratory.
Morrell, J.J.; Love, C.S.; Freitag, C.M. 1996. Integrated remedial protection of wood in bridges.
In: Ritter, M.A.; Duwadi, S.R.; Lee, P.D.H., ed(s). Proceedings for National Conference
on Wood Transportation Structures. Madison, WI. USDA, Forest Service, Forest
Products Laboratory.
Pilon, John, ed. 2002. Best management practices for the use of preservative-treated wood in
aquatic environments in Michigan. Roscommon, MI: Michigan Department of Natural
Resources.
Richards, M.J. and W.S. McNamara. 1997. The field performance of CCA-C treated sawn
refractory softwoods from North America. Proceedings for the International Research
Group on Wood Preservation Annual Meeting IRG/WP 97-40085. Whistler, Canada.
Ritter, M. A. 1992. Timber bridges- design, construction, inspection and maintenance.
Engineering Management Series EM7700-8. Washington, D.C. USDA, Forest Service.
Ross, R.J.; Pellerin, R.F.; Volny, N.; Salsig, W.W.; Falk, R.H. 1999. Inspection of timber bridges
using stress wave timing nondestructive evaluation tools-A guide for use and
interpretation. Gen. Tech. Rep. FPL-GTR-114. Madison, WI.: USDA, Forest Service,
Forest Products Laboratory.
Scheffer, T.C. 1971. A climate index for estimating potential for decay in wood structures above
ground. Forest Products Journal 21(9):25-31.
Silva, A. A., Love, C. S., Morrell, J. J. and DeGroot, R. C. 1999. Biocide protection of fielddrilled bolt holes in red oak, yellow poplar, loblolly pine, and Douglas-fir. Madison, WI:
USDA, Forest Service, Forest Products.
U.S. Department of Transportation Federal Highway Administration. 1995. Recording and
Coding Guide for the Structure Inventory and Appraisal of the Nation’s Bridges.
Washington, D.C. Office of Engineering Bridge Division, Bridge Management Branch.
Wacker, J. P., and Crawford, D. M., 2003. Extending service life of timber bridges with
preservatives. Proceedings for the 19th US-Japan Bridge Engineering Workshop, UJNR
Panel on Wind and Seismic Effects, Tsukuba, Japan.
White, D. J., Mekkawy, M., Klaiber, F. W., Wipf, T.J., 2007. Investigation of Steel-Stringer
Bridges: Superstructures and Substructures, Volume 2., Ames, IA.: Center for
Transportation Research and Education, Iowa State University.
Wikipedia Encyclopedia. 2007. http://wikipedia.org.
Wilcox, W.W. 1983. Sensitivity of the ‘Pick Test’ for field detection of early wood decay.
Forest Products Journal 33(2): 29-30.
77
WWPI. 2006. Best Management Practices for the Use of Treated Wood in Aquatic
Environments. Vancouver, WA. Western Wood Preservers Institute.
ADDITIONAL REFERENCES
Brooks, Kenneth M. 2000. Assessment of the environmental effects associated with wooden
bridges preserved with creosote, pentachlorophenol, or chromated copper arsenate. Res.
Pap. FPL–RP–587. Madison, WI: USDA Forest Service, Forest Products Laboratory.
Choi, S., J. N. R. Ruddick, and P. Morris. 2004. Chemical redistribution in CCA-treated decking.
Forest Products Journal.54(3):33–37.
Forest Products Laboratory. 2000. Environmental impact of preservative-treated wood in a
wetland boardwalk: Res. Pap. FPL-RP-582. Madison, WI: USDA Forest Service, Forest
Products Laboratory.
Goyette, D.; Brooks, K.M. 1998. Creosote evaluation: phase II. Sooke Basin study—baseline to
535 days post construction, 1995-1996. Report PR98–04. North Vancouver, British
Columbia, Canada: Environment Canada.
Jin, L.; Archer, K.; Preston, A.F. 1992. Depletion and biodeterioration studies with
developmental wood preservative formulations. Proceedings for the American Wood
Preservers’ Association.
Lebow, S.T., Hatfield, C.A.; Crawford, D.M. and B. Woodward. 2003. Long-term stake
evaluations of waterborne copper systems. Proceedings for the American Wood
Preservers Association Annual Meeting, Boston, MA.
Lebow, S. T. 1996. Leaching of wood preservative components and their mobility in the
environment—summary of pertinent literature. Gen. Tech. Rep. FPL–GTR–93. USDA
Forest Service, Forest Products Laboratory.
Lebow, S.T.; Halverson, S. A.; Morrell, J. J. and J. Simonsen, John. 2000. Role of construction
debris in release of copper, chromium, and arsenic from treated wood structures. Res.
Pap. FPL–RP–584. Madison, WI: USDA Forest Service, Forest Products Laboratory.
Lebow, S.T. and M. Tippie, Guide for minimizing the effect of preservative-treated wood on
sensitive environments. Gen. Tech. Rep. FPL–GTR–122. Madison, WI: USDA Forest
Service, Forest Products Laboratory.
Moore, H.B. 1979. Wood inhabiting insects in houses: their identification, biology and control.
Report prepared as part of interagency agreement IAA-25-75 between the USDA, Forest
Service and the Department of Housing and Urban Development.
Morris, P.I. 2000 Field testing of wood preservatives in Canada. X: A review of results.
Proceedings for the Annual Convention, Canadian Wood Preservation Association.
78
Pasek, E. 2003. Minimizing preservative losses: Fixation. A report of the P4
Migration/Depletion/Fixation Task Force. Proceedings for the American Wood
Preservers Association Annual Meeting, Boston, MA.
Ritter, M. A.; Duwadi, S. R. 1998. Research accomplishments for wood transportation
structures based on a national research needs assessment. Gen. Tech. Rep. FPL–GTR–
105. Madison, WI: USDA, Forest Service, Forest Products Laboratory.
Smith, D.N; Williams, A.I. 1969. Wood preservation by boron diffusion process-effect of
moisture content on diffusion time. Journal Institute of Wood Science 22 (4):3-10.
U.S. Department of Agriculture Forest Service, Forest Products Laboratory. 2000.
Environmental impact of preservative-treated wood in a wetland boardwalk. Res. Pap.
FPL–RP–582. Madison, WI: USDA Forest Service, Forest Products Laboratory.
79
A-0
APPENDIX A
IOWA COUNTY PRESERVATIVE QUESTIONNAIRE
A-1
To: Iowa County Engineers
A new research project – Field Evaluation of Timber Preservative Treatments for Iowa Highway
Applications TR-552 has been funded by the Iowa Highway Research Board and the Iowa DOT.
The primary objective of this project is to collect and document various timber preservative
treatments and develop an evaluation process that will permit bridge owners to make sound
decisions regarding their timber bridge treatment options.
The attached questionnaire is intended to assist the research team in collecting information
regarding current timber bridge preservation practices in Iowa. As appropriate, the collected
information will be used to supplement the recommendations and guidelines developed in this
project.
We recognize that you receive numerous surveys and questionnaires requesting various types of
information which all take a portion of your valuable time. With that in mind, we have designed
the questionnaire to be relatively simple and easy to complete. If you have any questions or
would prefer to provide input in another format, please contact either one of us.
In order to keep the project progressing on schedule, please complete the questionnaire and
return it to us by October 6, 2006 if at all possible. However, we would rather have your
response a few days late than not at all.
Thank you in advance for your assistance with this project. It is with your help that we hope to
produce a practical document that will assist county engineers, consultants, etc. with their timber
bridge preservative concerns.
Sincerely,
F. Wayne Klaiber
Professor of Civil Engineering
(515) 294-8763
[email protected]
Michael LaViolette
Bridge Engineering Specialist
(515) 294-6838
[email protected]
A-2
IOWA HIGHWAY RESEARCH BOARD
RESEARCH PROJECT TR-552
Field Evaluation of Timber Preservative Treatments for Iowa
Highway Applications
Questionnaire Completed by:
Organization:
Address:
Email:
Responses can be mailed, faxed or emailed to Wayne Klaiber or Mike LaViolette:
Wayne Klaiber
422 Town Engineering
Iowa State University
Ames, IA 50011
Phone (515) 294-8763
Fax (515) 294-7424
[email protected]
Michael LaViolette
ISU Bridge Engineering Center
2711 South Loop Drive, Suite 4700
Ames, IA 50010
Phone (515) 294-6838
Fax (515) 294-0467
[email protected]
We would appreciate a copy of any additional information you would be willing to
share, such as current specifications or timber bridge preservative information.
A-3
Section 1 – Timber Bridge Utilization
1.1. Does your county utilize timber in bridges, bridge components or other transportation
elements?
Yes
No
If you answered no, please skip to Section 3 of this questionnaire.
a. If yes, how many timber bridges do you currently have in inventory?
b. Would your county have any reservations with constructing a new timber bridge?
Yes
No
1.2. Regarding timber bridge backwalls/wingwalls:
a. Does your county have existing bridges with this feature?
Yes
No
b. Does your county construct new bridges with this feature?
Yes
No
c. If no, is there a particular reason?
d. If yes, are you willing to share particularly successful details?
1.3. Regarding timber bridge piling/substructure:
a. Does your county have existing bridges with this feature?
Yes
No
b. Does your county construct new bridges with this feature?
Yes
No
c. If no, is there a particular reason?
d. If yes, are you willing to share particularly successful details?
A-4
1.4. Regarding timber bridge superstructures:
a. Does your county have existing bridges with this feature?
Yes
No
b. Does your county construct new bridges with this feature?
Yes
No
c. If no, is there a particular reason?
d. If yes, are you willing to share particularly successful details?
1.5. Regarding timber bridge guardrail systems or sign posts:
a. Does your county have existing bridges with this feature?
Yes
No
b. Does your county construct new bridges with this feature?
Yes
No
c. If no, is there a particular reason?
d. If yes, are you willing to share particularly successful details?
Section 2 – Timber Bridge Preservatives
2.1. What types of shop-applied timber preservative treatment does your county currently
specify for timber bridges? (check all that apply)
ACZA
Copper HDO
ACC
Copper Naphthenate
ACQ
Creosote
CA-B
Oxine copper
CCA
Pentachlorophenol
a. Others not listed (possibly including trade names) ?
2.2. What type of preservative treatment does your county currently specify for field
applications for in-service structures?
A-5
2.2.1. Surface treatment:
Spray
Brush
Estimated Cost
Reapplication Schedule
2.2.2. Diffusible chemicals:
Boron rods
Flouride rods
Copper boron rods
Estimated cost
Reapplication Schedule
2.2.3. Fumigants:
Liquid
Granules
Restricted use
2.2.4. Others not listed (possibly including trade names)?
2.3. What method of application does your county currently specify for field treatment of
cuts, drilled holes, etc.?
2.4. Does your county currently specify a scheduled reapplication of preservative treatment?
Yes
No
If yes, how often?
2.5. What specifications does your county use for specifying preservative treatment?
State specifications
A-6
County has own specifications
AASHTO standards
AWPA standards
Other:
Section 3 – Timber Bridge Decision Making
3.1. What are the most important factors in your county’s decision not to use timber in
bridge components? Please rate these possible reasons in order of importance, with
one being the most important factor:
Cost
Durability concerns
Difficulty in specifying preservative treatment
Appearance
Odor or surface cleanliness (handling concerns)
Maintenance concerns
Material Availability
Just not accustomed to using timber
Ease of Installation
Strength properties
Concerns about corrosion of connectors
Other:
Other:
3.2. What do you see as the primary advantage(s) of or reasons your county might utilize
timber in bridge components? Please rate these possible advantages in order of
importance, with one being the most important factor.
Cost
Durability
Appearance
Maintenance
Material availability
Ease of installation
Strength properties
Other:
Other:
3.3. What are the primary factors that you consider when choosing a wood species for a
timber bridge components?
A-7
3.4. What are the primary factors that you consider when choosing a preservative treatment
for timber bridge components?
Section 4 – Timber Bridge Components Life Expectancy
For the questions in this section, please indicate an expected service life for timber bridge
components based on your experience. In addition, please indicate the most common form(s) of
deterioration observed that necessitate the replacement of these bridge components.
4.1. Timber Deck
1-5 years
6-10 years
11-15 years
16-20 years
21-25 years
26-30 years
31-50 years
Over 50 years
Most common form(s) of deterioration:
4.2. Timber Stringers
1-5 years
6-10 years
11-15 years
16-20 years
21-25 years
26-30 years
31-50 years
Over 50 years
Most common form(s) of deterioration:
4.3. Timber Piling
1-5 years
6-10 years
11-15 years
21-25 years
26-30 years
31-50 years
A-8
16-20 years
Over 50 years
Most common form(s) of deterioration:
4.4. Timber Backwall
1-5 years
6-10 years
11-15 years
16-20 years
21-25 years
26-30 years
31-50 years
Over 50 years
Most common form(s) of deterioration:
Section 5 – Timber Bridge Details
5.1. Does your county have timber bridges which feature details that have been especially
problematic?
Yes
No
5.2. Does your county have timber bridges which feature details that have been especially
successful?
Yes
No
5.3. If the answer to either of the above questions is “YES”, would you be willing to submit
detailed drawings or photos?
Yes
No
5.4. If the answer to either of the above questions is “YES”, would you be willing to permit
the bridge in question to be visited and reviewed by members of the research team?
Yes
No
A-9
Section 6 – Timber Bridge Inspection
6.1. Does your county (or a consultant hired by your county) perform scheduled inspection
of its timber bridges?
Yes
No
Consultants are hired to perform inspections
6.2. Does your county use any specific methods for detecting deterioration of timber
components?
External Deterioration
Internal Deterioration
Visual inspection
Sounding
Probing
Moisture meter
Pick test
Shigometer
Other
Drilling/Coring
Shell depth indicator
Sonic evaluation
X-ray tomography
Other
A-10
Fly UP